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SQL Commands – A Beginner’s Guide To SQL

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In the era where data is being generated in humongous amounts, there is a constant need to handle data in databases. Relational databases are one of the most popular databases, and SQL is the basis of relational databases. Therefore  SQL skills are indispensable in most of the job roles. In this article on SQL Commands, I will discuss the top commands and statements that you need to understand in SQL.

The topics covered in this blog are mainly divided into 4 categories:

  • Data Definition Language(DDL) – Consists of commands which are used to define the database.
  • Data Manipulation Language(DML) –  Consists of commands which are used to manipulate the data present in the database.
  • Data Control Language(DCL) – Consists of commands which deal with the user permissions and controls of the database system.
  • Transaction Control Language(TCL) – Consist of commands which deal with the transaction of the database.

Apart from the above commands, the following topics will also be covered in this article:

  • Different Types Of Keys In Database
  • Constraints Used In Database
  • Dates & Auto Increment

In this article on SQL Commands, I am going to consider the below database asan example, to show you how to writecommands.

EmployeeID EmployeeName Emergency ContactName PhoneNumber Address City Country 01 Shanaya Abhinay 9898765612 Oberoi Street 23 Mumbai India 02 Anay Soumya 9432156783 Marathalli House No 23 Delhi India 03 Preeti Rohan 9764234519 Queens Road 45 Bangalore India 04 Vihaan Akriti 9966442211 Brigade Road Block 4 Hyderabad India 05 Manasa Shourya 9543176246 Mayo Road 23 Kolkata India

So, let’s get started now!

Comments in SQL

There are two ways in which you can comment in SQL, i.e. either theor the.

Single-Line Comments

The single line comment starts with two hyphens (–). So, any text mentioned after (–), till the end of a single line will be ignored by the compiler.

Example:

--Select all:
SELECT * FROM Employee_Info;

Multi-Line Comments  

The Multi-line comments start with /* and end with */ . So, any text mentioned between /* and */ will be ignored by the compiler.

Example:

/*Select all the columns
of all the records
from the Employee_Info table:*/
SELECT * FROM Students;

SQL Commands: Data Definition Language Commands (DDL)

This section of the article will give you an insight into the commands through which you can define your database. The commands are as follows:

CREATE

This statement is used to create a table or a database.

The ‘CREATE DATABASE’ Statement

As the name suggests, this statement is used to create a database.

Syntax

CREATE DATABASE DatabaseName;

Example

CREATE DATABASE Employee;

The ‘CREATE TABLE’ Statement

This statement is used to create a table.

Syntax

CREATE TABLE TableName (
Column1 datatype,
Column2 datatype,
Column3 datatype,
....

ColumnN datatype
);

Example

CREATE TABLE Employee_Info
(
EmployeeID int,
EmployeeName varchar(255),
Emergency ContactName varchar(255),
PhoneNumber int,
Address varchar(255),
City varchar(255),
Country varchar(255)
);

You can also create a table using another table. Refer the below sytax and example:

The ‘CREATE TABLE AS’ Statement

CREATE TABLE NewTableName AS
SELECT Column1, column2,..., ColumnN
FROM ExistingTableName
WHERE ....;

Example

CREATE TABLE ExampleTable AS
SELECT EmployeeName, PhoneNumber
FROM Employee_Info;

DROP

This statement is used to drop an existing table or a database.

The ‘DROP DATABASE’ Statement

This statement is used to drop an existing database. When you use this statement, complete information present in the database will be lost.

Syntax

DROP DATABASE DatabaseName;

Example

DROP DATABASE Employee;

The ‘DROP TABLE’ Statement

This statement is used to drop an existing table. When you use this statement, complete information present in the table will be lost.

Syntax

DROP TABLE TableName;

Example

DROP Table Employee_Info;

TRUNCATE

This command is used to delete the information present in the table but does not delete the table. So, once you use this command, your information will be lost, but not the table.

Syntax

TRUNCATE TABLE TableName;

Example

TRUNCATE Table Employee_Info;

ALTER

This command is used to delete, modify or add constraints or columns in an existing table.

The ‘ALTER TABLE’ Statement

This statement is used to add, delete, modify columns in an existing table.

The ‘ALTER TABLE’ Statement with ADD/DROP COLUMN

You can use the ALTER TABLE statement with ADD/DROP Column command according to your need. If you wish to add a column, then you will use the ADD command, and if you wish to delete a column, then you will use the DROP COLUMN command.

Syntax

ALTER TABLE TableName
ADD ColumnName Datatype;

ALTER TABLE TableName
DROP COLUMN ColumnName;

Example

--ADD Column BloodGroup:

ALTER TABLE Employee_Info
ADD BloodGroup varchar(255);

--DROP Column BloodGroup:

ALTER TABLE Employee_Info
DROP COLUMN BloodGroup ;

The ‘ALTER TABLE’ Statement with ALTER/MODIFY COLUMN

This statement is used to change the datatype of an existing column in a table.

ALTER TABLE TableName
ALTER COLUMN ColumnName Datatype;

Example

--Add a column DOB and change the data type to Date.

ALTER TABLE Employee_Info
ADD DOB year;

ALTER TABLE Employee_Info
ALTER DOB date;

BACKUP DATABASE

This statement is used to create a full backup of an existing database.

Syntax

BACKUP DATABASE DatabaseName
TO DISK = 'filepath';

Example

BACKUP DATABASE Employee
TO DISK = 'C:\Users\Sahiti\Desktop';

You can also use a differential back up. This type of back up only backs up the parts of the database, which have changed since the last complete backup of the database.

Syntax

BACKUP DATABASE DatabaseName
TO DISK = 'filepath'
WITH DIFFERENTIAL;

Example

BACKUP DATABASE Employee
TO DISK = 'C:\Users\Sahiti\Desktop'
WITH DIFFERENTIAL;

Now that you know the data definition commands, let me take you through the various types of Keys and Constraints that you need to understand before learning how to manipulate the databases.

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SQL Commands: Different Types Of Keys In Database

There are mainly 7 types of Keys, that can be considered in a database. I am going to consider the below tables to explain to you the various keys.

veI3uin.png!web

  • Candidate Key – A set of attributes which can uniquely identify a table can be termed as a Candidate Key. A table can have more than one candidate key, and out of the chosen candidate keys, one key can be chosen as a Primary Key. In the above example, since EmployeeID, InsuranceNumber and PanNumber can uniquely identify every tuple, they would be considered as a Candidate Key. 
  • Super Key –  The set of attributes which can uniquely identify a tuple is known as Super Key. So, a candidate key, primary key, and a unique key is a superkey, but vice-versa isn’t true.
  • Primary Key –  A set of attributes which are used to uniquely identify every tuple is also a primary key. In the above example, since EmployeeID, InsuranceNumber and PanNumber are candidate keys, any one of them can be chosen as a Primary Key. Here EmployeeID is chosen as the primary key.
  • Alternate Key – Alternate Keys are the candidate keys, which are not chosen as a Primary key. From the above example, the alternate keys are PanNumber and Insurance Number.
  • Unique Key   The unique key is similar to the primary key, but allows one NULL value in the column. Here the Insurance Number and the Pan Number can be considered as unique keys.
  • Foreign Key –  An attribute that can only take the values present as the values of some other attribute, is the foreign key to the attribute to which it refers. in the above example, the Employee_ID from the Employee_Information Table is referred to the Employee_ID from the Employee_Salary Table.
  • Composite Key  A composite key is a combination of two or more columns that identify each tuple uniquely. Here, the Employee_ID and Month-Year_Of_Salary can be grouped together to uniquely identify every tuple in the table.

SQL Commands: Constraints Used In Database

Constraints are used in a database to specify the rules for data in a table. The following are the different types of constraints:

NOT NULL

This constraint ensures that a column cannot have a NULL value.

Example

--NOT NULL on Create Table

CREATE TABLE Employee_Info
(
EmployeeID int NOT NULL,
EmployeeName varchar(255) NOT NULL,
Emergency ContactName varchar(255),
PhoneNumber int NOT NULL,
Address varchar(255),
City varchar(255),
Country varchar(255)
);

--NOT NULL on ALTER TABLE

ALTER TABLE Employee_Info
MODIFY PhoneNumber int NOT NULL;

UNIQUE

This constraint ensures that all the values in a column are unique.

Example

--UNIQUE on Create Table

CREATE TABLE Employee_Info
(
EmployeeID int NOT NULL UNIQUE,
EmployeeName varchar(255) NOT NULL,
Emergency ContactName varchar(255),
PhoneNumber int NOT NULL,
Address varchar(255),
City varchar(255),
Country varchar(255)
);

--UNIQUE on Multiple Columns

CREATE TABLE Employee_Info
(
EmployeeID int NOT NULL,
EmployeeName varchar(255) NOT NULL,
Emergency ContactName varchar(255),
PhoneNumber int NOT NULL,
Address varchar(255),
City varchar(255),
Country varchar(255),
CONSTRAINT UC_Employee_Info UNIQUE(Employee_ID, PhoneNumber)
);

--UNIQUE on ALTER TABLE

ALTER TABLE Employee_Info
ADD UNIQUE (Employee_ID);

--To drop a UNIQUE constraint

ALTER TABLE  Employee_Info
DROP CONSTRAINT UC_Employee_Info;

CHECK

This constraint ensures that all the values in a column satisfy a specific condition.

Example

--CHECK Constraint on CREATE TABLE

CREATE TABLE Employee_Info
(
EmployeeID int NOT NULL,
EmployeeName varchar(255),
Emergency ContactName varchar(255),
PhoneNumber int,
Address varchar(255),
City varchar(255),
Country varchar(255) CHECK (Country=='India')
);

--CHECK Constraint on multiple columns

CREATE TABLE Employee_Info
(
EmployeeID int NOT NULL,
EmployeeName varchar(255),
Emergency ContactName varchar(255),
PhoneNumber int,
Address varchar(255),
City varchar(255),
Country varchar(255) CHECK (Country = 'India' AND Cite = 'Hyderabad')
);

--CHECK Constraint on ALTER TABLE

ALTER TABLE Employee_Info
ADD CHECK (Country=='India');

--To give a name to the CHECK Constraint

ALTER TABLE Employee_Info
ADD CONSTRAINT CheckConstraintName CHECK (Country=='India');

--To drop a CHECK Constraint

ALTER TABLE Employee_Info
DROP CONSTRAINT CheckConstraintName;

DEFAULT

This constraint consists of a set of default values for a column when no value is specified.

Example

--DEFAULT Constraint on CREATE TABLE

CREATE TABLE Employee_Info
(
EmployeeID int NOT NULL,
EmployeeName varchar(255),
Emergency ContactName varchar(255),
PhoneNumber int,
Address varchar(255),
City varchar(255),
Country varchar(255) DEFAULT 'India'
);

--DEFAULT Constraint on ALTER TABLE

ALTER TABLE Employee_Info
ADD CONSTRAINT defau_Country
DEFAULT 'India' FOR Country;

--To drop the Default Constraint

ALTER TABLE Employee_Info
ALTER COLUMN Country DROP DEFAULT;

INDEX

This constraint is used to create indexes in the table, through which you can create and retrieve data from the database very quickly.

--Create an Index where duplicate values are allowed
CREATE INDEX IndexName
ON TableName (Column1, Column2, ...ColumnN);

--Create an Index where duplicate values are not allowed
CREATE UNIQUE INDEX IndexName
ON TableName (Column1, Column2, ...ColumnN);

Example

CREATE INDEX idex_EmployeeName
ON Persons (EmployeeName);

--To delete an index in a table

DROP INDEX Employee_Info.idex_EmployeeName;

Now, let us look into the next part of this article i.e. DML Commands.

SQL Commands: Data Manipulation Language Commands (DML)

This section of the article will give you an insight into the commands through which you can manipulate the database. The commands are as follows:

Apart from these commands, there are also other manipulative operators/functions such as:

USE

The USE statement is used to select the database on which you want to perform operations.

Syntax

USE DatabaseName;

Example

USE Employee;

INSERT INTO

This statement is used to insert new records into the table.

Syntax

INSERT INTO TableName (Column1, Column2, Column3, ...,ColumnN)
VALUES (value1, value2, value3, ...);

--If you don't want to mention the column names then use the below syntax

INSERT INTO TableName
VALUES (Value1, Value2, Value3, ...);

Example

INSERT INTO Employee_Info(EmployeeID, EmployeeName, Emergency ContactName, PhoneNumber, Address, City, Country)
VALUES ('06', 'Sanjana','Jagannath', '9921321141', 'Camel Street House No 12', 'Chennai', 'India');

INSERT INTO Employee_Info
VALUES ('07', 'Sayantini','Praveen', '9934567654', 'Nice Road 21', 'Pune', 'India');

UPDATE

This statement is used to modify the records already present in the table.

UPDATE TableName
SET Column1 = Value1, Column2 = Value2, ...
WHERE Condition;

Example

UPDATE Employee_Info
SET EmployeeName = 'Aahana', City= 'Ahmedabad'
WHERE EmployeeID = 1;

This statement is used to delete the existing records in a table.

Syntax

DELETE FROM TableName WHERE Condition;

Example

DELETE FROM Employee_Info
WHERE EmployeeName='Preeti';

SELECT

This statement is used to select data from a database and the data returned is stored in a result table, called the  result-set .

Syntax

SELECT Column1, Column2, ...ColumN
FROM TableName;

--(*) is used to select all from the table
SELECT * FROM table_name;

-- To select the number of records to return use:
SELECT TOP 3 * FROM TableName;

Example

SELECT EmployeeID, EmployeeName
FROM Employee_Info;

--(*) is used to select all from the table
SELECT * FROM Employee_Info;

-- To select the number of records to return use:
SELECT TOP 3 * FROM Employee_Info;

Apart from just using the SELECT keyword individually, you can use the following keywords with the SELECT statement:

The ‘SELECT DISTINCT’ Statement

This statement is used to return only different values.

SELECT DISTINCT Column1, Column2, ...ColumnN
FROM TableName;

Example

SELECT DISTINCT PhoneNumber FROM Employee_Info;

The ‘ORDER BY’ Statement

The ‘ORDER BY’ statement is used to sort the required results in ascending or descending order. The results are sorted in ascending order by default. Yet, if you wish to get the required results in descending order, you have to use the DESC keyword.

Syntax

SELECT Column1, Column2, ...ColumnN
FROM TableName
ORDER BY Column1, Column2, ... ASC|DESC;

Example

-- Select all employees from the 'Employee_Info' table sorted by EmergencyContactName:
SELECT * FROM Employee_Info
ORDER BY EmergencyContactName;

-- Select all employees from the 'Employee_Info' table sorted by EmergencyContactName in Descending order:
SELECT * FROM Employee_Info
ORDER BY EmergencyContactName DESC;

-- Select all employees from the 'Employee_Info' table sorted by EmergencyContactName and EmployeeName:
SELECT * FROM Employee_Info
ORDER BY EmergencyContactName, EmployeeName;

/* Select all employees from the 'Employee_Info' table sorted by EmergencyContactName in Descending order and EmployeeName in Ascending order: */
SELECT * FROM Employee_Info
ORDER BY EmergencyContactName ASC, EmployeeName DESC;

The ‘GROUP BY’ Statement

This ‘GROUP BY’ statement is used with the aggregate functions to group the result-set by one or more columns.

SELECT Column1, Column2,..., ColumnN
FROM TableName
WHERE Condition
GROUP BY ColumnName(s)
ORDER BY ColumnName(s);

Example

-- To list the number of employees from each city.

SELECT COUNT(EmployeeID), City
FROM Employee_Info
GROUP BY City;

The ‘HAVING’ Clause

The ‘HAVING’ clause is used in SQL because the WHERE keyword cannot be used everywhere.

Syntax

SELECT ColumnName(s)
FROM TableName
WHERE Condition
GROUP BY ColumnName(s)
HAVING Condition
ORDER BY ColumnName(s);

Example

/*  To list the number of employees in each city. The employees should be sorted high to low and only those cities must be included who have more than 5 employees:*/

SELECT COUNT(EmployeeID), City
FROM Employee_Info
GROUP BY City
HAVING COUNT(EmployeeID) > 2
ORDER BY COUNT(EmployeeID) DESC;

The ‘SELECT INTO’ Statement

The ‘SELECT INTO’ statement is used to copy data from one table to another.

Syntax

SELECT *
INTO NewTable [IN ExternalDB]
FROM OldTable
WHERE Condition;

Example

-- To create a backup of database 'Employee'
SELECT * INTO EmployeeBackup
FROM Employee;

--To select only few columns from Employee
SELECT EmployeeName, PhoneNumber INTO EmployeeContactDetails
FROM Employee;

SELECT * INTO BlrEmployee
FROM Employee
WHERE City = 'Bangalore';

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Now, as I mentioned before, let us move onto our next section in this article on SQL Commands, i.e. the Operators.

Operators in SQL

The different set of operators available in SQL are as follows:

nu2umyM.png!web

Let us look into each one of them, one by one.

Arithmetic Operators

Operator Description % Modulous [A % B] / Division [A / B] * Multiplication [A * B] – Subtraction  [A – B] + Addition [A + B]

Bitwise Operators

Operator Description ^ Bitwise Exclusive OR (XOR) [A ^ B] | Bitwise OR [A | B] & Bitwise AND [A & B]

Comparison Operators

Operator Description < > Not Equal to [A < > B] <= Less than or equal to [A <= B] >= Greater than or equal to [A >= B] < Less than [A < B] > Greater than [A > B] = Equal to [A = B]

Compound Operators

Operator Description |*= Bitwise OR equals [A |*= B] ^-= Bitwise Exclusive equals  [A ^-= B] &= Bitwise AND equals [A &= B] %= Modulo equals [A %= B] /= Divide equals [A /= B] *= Multiply equals [A*= B] -= Subtract equals [A-= B] += Add equals [A+= B]

Logical Operators

The Logical operators present in SQL are as follows:

AND Operator

This operator is used to filter records that rely on more than one condition. This operator displays the records, which satisfy all the conditions separated by AND, and give the output TRUE.

Syntax

SELECT Column1, Column2, ..., ColumnN
FROM TableName
WHERE Condition1 AND Condition2 AND Condition3 ...;

Example

SELECT * FROM Employee_Info
WHERE City='Mumbai' AND City='Hyderabad';</pre>

OR Operator

This operator displays all those records which satisfy any of the conditions separated by OR and give the output TRUE.

SELECT Column1, Column2, ..., ColumnN
FROM TableName
WHERE Condition1 OR Condition2 OR Condition3 ...;

Example

SELECT * FROM Employee_Info
WHERE City='Mumbai' OR City='Hyderabad';

NOT Operator

The NOT operator is used, when you want to display the records which do not satisfy a condition.

Syntax

SELECT Column1, Column2, ..., ColumnN
FROM TableName
WHERE NOT Condition;

Example

SELECT * FROM Employee_Info
WHERE NOT City='Mumbai';

NOTE:You can also combine the above three operators and write a query as follows:

SELECT * FROM Employee_Info
WHERE NOT Country='India' AND (City='Bangalore' OR City='Hyderabad');

NOTE:You can also combine the above three operators and write a query as follows:

SELECT * FROM Employee_Info
WHERE NOT Country='India' AND (City='Bangalore' OR City='Hyderabad');

BETWEEN Operator

The BETWEEN operator is used, when you want to select values within a given range. Since this is an inclusive operator, both the starting and ending values are considered.

SELECT ColumnName(s)
FROM TableName
WHERE ColumnName BETWEEN Value1 AND Value2;

Example

SELECT * FROM Employee_Salary
WHERE Salary BETWEEN 40000 AND 50000;

LIKE Operator

The LIKE operator is used in a WHERE clause to search for a specified pattern in a column of a table. There are mainly two wildcards that are used in conjunction with the LIKE operator:

  • % – It matches 0 or more character.
  • _ – It matches exactly one character.

Syntax

SELECT ColumnName(s)
FROM TableName
WHERE ColumnName LIKE pattern;

Refer to the following table for the various patterns that you can mention with the LIKE operator.

Like Operator Condition Description WHERE CustomerName LIKE ‘v% Finds any values that start with “v” WHERE CustomerName LIKE ‘%v’ Finds any values that end with “v” WHERE CustomerName LIKE ‘%and%’ Finds any values that have “and” in any position WHERE CustomerName LIKE ‘_q%’ Finds any values that have “q” in the second position. WHERE CustomerName LIKE ‘u_%_%’ Finds any values that start with “u” and are at least 3 characters in length WHERE ContactName LIKE ‘m%a’ Finds any values that start with “m” and end with “a”

Example

SELECT * FROM Employee_Info
WHERE EmployeeName LIKE 'S%';

IN Operator

This operator is used for multiple OR conditions. This allows you to specify multiple values in a WHERE clause.

SELECT ColumnName(s)
FROM TableName
WHERE ColumnName IN (Value1,Value2...);

Example

SELECT * FROM Employee_Info
WHERE City IN ('Mumbai', 'Bangalore', 'Hyderabad');

NOTE:  You can also use IN while writing  .

EXISTS Operator

The EXISTS operator is used to test if a record exists or not.

SELECT ColumnName(s)
FROM TableName
WHERE EXISTS
(SELECT ColumnName FROM TableName WHERE condition);

Example

SELECT EmergencyContactName
FROM Employee_Info
WHERE EXISTS (SELECT EmergencyContactName FROM Employee_Info WHERE EmployeeId = 05 AND City = 'Kolkata');

ALL Operator

The ALL operator is used with a WHERE orand returns TRUE if all of the subquery values meet the condition.

SELECT ColumnName(s)
FROM TableName
WHERE ColumnName operator ALL
(SELECT ColumnName FROM TableName WHERE condition);

Example

SELECT EmployeeName
FROM Employee_Info
WHERE EmployeeID = ALL (SELECT EmployeeID FROM Employee_Info WHERE City = 'Hyderabad');

ANY Operator

Similar to the ALL operator, the ANY operator is also used with a WHERE orand returns true if any of the subquery values meet the condition.

Syntax

SELECT ColumnName(s)
FROM TableName
WHERE ColumnName operator ANY
(SELECT ColumnName FROM TableName WHERE condition);

Example

SELECT EmployeeName
FROM Employee_Info
WHERE EmployeeID = ANY (SELECT EmployeeID FROM Employee_Info WHERE City = 'Hyderabad' OR City = 'Kolkata');

Next, in this article on SQL Commands, let us look into the various Aggregate Functions provided in SQL.

Aggregate Functions

This section of the article will include the following functions:

MIN() Function

The MIN function returns the smallest value of the selected column in a table.

SELECT MIN(ColumnName)
FROM TableName
WHERE Condition;

Example

SELECT MIN(EmployeeID) AS SmallestID
FROM Employee_Info;

MAX() Function

The MAX function returns the largest value of the selected column in a table.

SELECT MAX(ColumnName)
FROM TableName
WHERE Condition;

Example

SELECT MAX(Salary) AS LargestFees
FROM Employee_Salary;

COUNT() Function

The COUNT function returns the number of rows which match the specified criteria.

SELECT COUNT(ColumnName)
FROM TableName
WHERE Condition;

Example

SELECT COUNT(EmployeeID)
FROM Employee_Info;

SUM() Function

The SUM function returns the total sum of a numeric column that you choose.

Syntax

SELECT SUM(ColumnName)
FROM TableName
WHERE Condition;

Example

SELECT SUM(Salary)
FROM Employee_Salary;

AVG() Function

The AVG function returns the average value of a numeric column that you choose.

SELECT AVG(ColumnName)
FROM TableName
WHERE Condition;

Example

SELECT AVG(Salary)
FROM Employee_Salary;

NULL Functions

The NULL functions are those functions which let you return an alternative value if an expression is NULL. In the SQL Server, the function is ISNULL() .

Example

SELECT EmployeeID * (Month_Year_of_Salary + ISNULL(Salary, 0))
FROM Employee_Salary;

Aliases & Case Statement

In this section of this article on SQL Commands, you will go through theandone after the other.

Aliases

Aliases are used to give a column/table a temporary name and only exists for a duration of the query.

Syntax

--Alias Column Syntax

SELECT ColumnName AS AliasName
FROM TableName;

--Alias Table Syntax

SELECT ColumnName(s)
FROM TableName AS AliasName;

Example

SELECT EmployeeID AS ID, EmployeeName AS EmpName
FROM Employee_Info;

SELECT EmployeeName AS EmpName, EmergencyContactName AS [Contact Name]
FROM Employee_Info;

Case Statement

This statement goes through all the conditions and returns a value when the first condition is met. So, if no conditions are TRUE, it returns the value in the ELSE clause. Also, if no conditions are true and there is no ELSE part, then it returns NULL.

Syntax

CASE
WHEN Condition1 THEN Result1
WHEN Condition2 THEN Result2
WHEN ConditionN THEN ResultN
ELSE Result
END;

Example

SELECT EmployeeName, City
FROM Employee_Info
ORDER BY
(CASE
    WHEN City IS NULL THEN 'Country is India by default'
    ELSE City
END);

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Now, that I have told you a lot about DML commands in this article on SQL Commands, let me just tell you in short about  , andDates & Auto Increment .

SQL Commands: Nested Queries

Nested queries   are those queries which have an outer query and inner subquery. So, basically, the subquery is a query which is nested within another query such as,,or. Refer to the image below:

uUVN7jf.png!web SQL Commands: Joins

JOINS are used to combine rows from two or more tables, based on a related column between those tables. The following are the types of joins: 

  •  This join returns those records which have matching values in both the tables.
  •  This join returns all those records which either have a match in the left or the right table.
  •  This join returns records from the left table, and also those records which satisfy the condition from the right table.
  •  This join returns records from the right table, and also those records which satisfy the condition from the left table.

Refer to the image below.

eUF7Vnf.png!web

Let’s consider the below table apart from the Employee_Info table, to understand the syntax of joins.

TechID EmpID TechName ProjectStartDate 1 10 DevOps 04-01-2019 2 11 Blockchain 06-07-2019 3 12 Python 01-03-2019

INNER JOIN

Syntax

SELECT ColumnName(s)
FROM Table1
INNER JOIN Table2 ON Table1.ColumnName = Table2.ColumnName;

Example

SELECT Technologies.TechID, Employee_Info.EmployeeName
FROM Technologies
INNER JOIN Employee_Info ON Technologies.EmpID = Employee_Info.EmpID;

FULL JOIN

SELECT ColumnName(s)
FROM Table1
FULL OUTER JOIN Table2 ON Table1.ColumnName = Table2.ColumnName;

Example

SELECT Employee_Info.EmployeeName, Technologies.TechID
FROM Employee_Info
FULL OUTER JOIN Orders ON Employee_Info.EmpID=Employee_Salary.EmpID
ORDER BY Employee_Info.EmployeeName;

LEFT JOIN

SELECT ColumnName(s)
FROM Table1
LEFT JOIN Table2 ON Table1.ColumnName = Table2.ColumnName;

Example

SELECT Employee_Info.EmployeeName, Technologies.TechID
FROM Employee_Info
LEFT JOIN Technologies ON Employee_Info.EmployeeID = Technologies.EmpIDID
ORDER BY Employee_Info.EmployeeName;

RIGHT JOIN

SELECT ColumnName(s)
FROM Table1
RIGHT JOIN Table2 ON Table1.ColumnName = Table2.ColumnName;

Example

SELECT Technologies.TechID
FROM Technologies
RIGHT JOIN Employee_Info ON Technologies.EmpID = Employee_Info.EmployeeID
ORDER BY Technologies.TechID;

SQL Commands: Set Operations

There are mainly three set operations:, ,. You can refer to the image below to understand the set operations in SQL.

numa6f6.png!web

UNION

This operator is used to combine the result-set of two or morestatements.

Syntax

SELECT ColumnName(s) FROM Table1
UNION
SELECT ColumnName(s) FROM Table2;

INTERSECT

This clause used to combine two   statements and return the intersection of the data-sets of both the SELECT statements.

SELECT Column1 , Column2 ....
FROM TableName
WHERE Condition

INTERSECT

SELECT Column1 , Column2 ....
FROM TableName
WHERE Condition

EXCEPT

This operator returns those tuples that are returned by the firstoperation, and are not returned by the second SELECT operation.

Syntax

SELECT ColumnName
FROM TableName

EXCEPT

SELECT ColumnName
FROM TableName;

Next, in this article, let us look into the date functions and auto-increment fields.

SQL Commands: Dates & Auto Increment

In this section of this article, I will explain to you how to use theand also thefields.

Dates

The following data types are present in a SQL Server to store a date or a date/time value in a database.

 Data Type  Format DATE YYYY-MM-DD DATETIME YYYY-MM-DD HH:MI:SS SMALLDATETIME YYYY-MM-DD HH:MI:SS TIMESTAMP A Unique Number

Example

SELECT * FROM Technologies WHERE ProjectStartDate='2019-04-01'

Auto Increment

This field generates a unique number automatically when a new record is inserted into a table. The MS SQL Server uses the IDENTITY keyword for this feature.

Example

/* To define the "CustId" column to be an auto-increment primary key field in the "Customers" table */

CREATE TABLE Customers (
CustId INT IDENTITY(1,1) PRIMARY KEY,
FirstName VARCHAR(255) NOT NULL
LastName VARCHAR(255) NOT NULL,
Age INT
);

Now, that you guys know the DML commands, let’s move onto our next sectionin this article on SQL Commands i.e. the DCL commands.

SQL Commands: Data Control Language Commands (DCL)

This section of the article will give you an insight into the commands which are used to enforce database security in multiple user database environments. The commands are as follows:

GRANT

This command is used to provide access or privileges on the database and its objects to the users.

Syntax

GRANT PrivilegeName
ON ObjectName
TO {UserName |PUBLIC |RoleName}
[WITH GRANT OPTION];

where,

  • PrivilegeName – Is the privilege/right/access granted to the user.
  • ObjectName – Name of a database object like TABLE/VIEW/STORED PROC.
  • UserName – Name of the user who is given the access/rights/privileges.
  • PUBLIC – To grant access rights to all users.
  • RoleName – The name of a set of privileges grouped together.
  • WITH GRANT OPTION – To give the user access to grant other users with rights.

Example

-- To grant SELECT permission to Employee_Info table to user1
GRANT SELECT ON Employee_Info TO user1;

REVOKE

This command is used to withdraw the user’s access privileges given by using thecommand.

Syntax

REVOKE PrivilegeName 
ON ObjectName 
FROM {UserName |PUBLIC |RoleName}

Example

-- To revoke the granted permission from user1
REVOKE SELECT ON Employee_Info TO user1;

Now, next in this article on SQL Commands, I will discuss,, and.

SQL Commands: Views

A view in SQL is a single table, which is derived from other tables. So, a view contains rows and columns similar to a real table and has fields from one or more table.

qY3aQvj.png!web

The ‘CREATE VIEW’ statement

This statement is used to create a view, from a table.

Syntax

CREATE VIEW ViewName AS
SELECT Column1, Column2, ..., ColumnN
FROM TableName
WHERE Condition;

Example

CREATE VIEW [Kolkata Employees] AS
SELECT EmployeeName, PhoneNumber
FROM Employee_Info
WHERE City = "Kolkata";

The ‘CREATE OR REPLACE VIEW’ statement

This statement is used to update a view.

Syntax

CREATE VIEW OR REPLACE ViewName AS
SELECT Column1, Column2, ..., ColumnN
FROM TableName
WHERE Condition;

Example

CREATE VIEW OR REPLACE [Kolkata Employees] AS
SELECT EmployeeName, PhoneNumber
FROM Employee_Info
WHERE City = "Kolkata";

The ‘DROP VIEW’ statement

This statement is used to delete a view.

Syntax

DROP VIEW ViewName;

Example

DROP VIEW [Kolkata Employees];

SQL Commands: Stored Procedures

A code which you can save and reuse it again is known as StoredProcedures.

CREATE PROCEDURE ProcedureName
AS
SQLStatement
GO;

Example

EXEC ProcedureName;

SQL Commands: Triggers

Triggers are a set of  SQL statements which are stored in the database catalog. These statements are executed whenever an event associated with a table occurs. So, a trigger can be invoked either BEFORE or AFTER the data is changed by  INSERT , UPDATE or DELETE statement. Refer to the image below.

2euqeyy.png!web

CREATE TRIGGER [TriggerName]
[BEFORE | AFTER]
{INSERT | UPDATE | DELETE}
on [TableName]
[FOR EACH ROW]
[TriggerBody]

Now, let’s move on to the last section of this article on SQL Commands i.e. the Transaction Control Language Commands.

SQL Commands: Transaction Control Language Commands (TCL)

This section of the article will give you an insight into the commands which are used to manage transactions in the database.The commands are as follows:

COMMIT

This command is used to save the transaction into the database.

Syntax

COMMIT;

ROLLBACK

This command is used to restore the database to the last committed state. 

ROLLBACK;

NOTE:When you use ROLLBACK with SAVEPOINT, then you can directly jump to a savepoint in an ongoing transaction.

Syntax:  ROLLBACK TO SavepointName;

SAVEPOINT

This command is used to temporarily save a transaction.  So if you wish to rollback to any point, then you can save that point as a ‘SAVEPOINT’.

Syntax

SAVEPOINT SAVEPOINTNAME;

Consider the below example to understand the working of transactions in the database.

EmployeeID EmployeeName 01 Ruhaan 02 Suhana 03 Aayush 04 Rashi

Now, use the below SQL queries to understand the transactions in the database.

INSERT INTO Employee_Table VALUES(05, 'Avinash');
COMMIT;
UPDATE Employee_Table SET name = 'Akash' WHERE id = '05';
SAVEPOINT S1;
INSERT INTO Employee_Table VALUES(06, 'Sanjana');
SAVEPOINT S2;
INSERT INTO Employee_Table VALUES(07, 'Sanjay');
SAVEPOINT S3;
INSERT INTO Employee_Table VALUES(08, 'Veena');
SAVEPOINT S4;
SELECT * FROM Employee_Table;

The output to the above set of queries would be as follows:

EmployeeID EmployeeName 01 Ruhaan 02 Suhana 03 Aayush 04 Rashi 05 Akash 06 Sanjana 07 Sanjay 08 Veena

Now, if you rollback to S2 using the below queries, the output is mentioned in the below table.

ROLLBACK TO S2;
SELECT * FROM Employee_Table;
EmployeeID EmployeeName 01 Ruhaan 02 Suhana 03 Aayush 04 Rashi 05 Akash 06 Sanjana

By this, I come to the end of this article on SQL Commands. I hope you enjoyed reading this article on SQL Commands. We have seen the different commands that will help you write queries and play around with your databases. If you wish to learn more aboutMySQL and get to know this open source relational database, then check out our   MySQL DBA Certification Training  which comes with instructor-led live training and real-life project experience. This training will help you understand MySQL in depth and help you achieve mastery over the subject.

Got a question for us? Please mention it in the comments section of ” SQL Commands ” and I will get back to you.

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