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SQL Joins Explained Using Pseudo (Python) Code

 4 years ago
source link: https://snakeycode.wordpress.com/2019/08/01/sql-joins-explained-using-pseudo-python-code/
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SQL Joins Explained Using Pseudo (Python) Code

There are many SQL JOIN tutorials that explain joins using Venn diagrams and/or very clean, sensible data. For me, those tutorials are not very intuitive. So here is my attempt at explaining them using Python as pseudo-code. Obviously in real code, you would almost always be better off using a real database (e.g. sqlite) with SQL.

# All these examples have been tested using SQL in Postgresql
# pseudo tables each with 2 cols (they do not have to have the same number of columns)
table1 = [[1, 'A'], [1, 'B'], [2, 'C'], [2, 'A'], [3, 'D'], [4, 'E'], [5, 'E']]
table2 = [[1, 'w'], [1, 'x'], [2, 'w'], [2, 'y'], [3, 'z'], [6, 's'], [7, 's']]
def do_join(join_type):
print('\n---------------------------------------------------------------')
print(join_type)
print('SELECT table1.col2, table2.col2 FROM table1 {} table2 ON table1.col1=table2.col1;'.format(join_type))
table1_used = set()
table2_used = set()
results = []
for id1, row1 in enumerate(table1):
for id2, row2 in enumerate(table2):
if row1[0] == row2[0]:
results.append([id1, id2] + row1[1:] + row2[1:])
table1_used.add(id1)
table2_used.add(id2)
if join_type in ['FULL JOIN', 'LEFT JOIN']:
none_cols = [None] * (len(table2[0]) - 1)
for id1, row1 in enumerate(table1):
if id1 not in table1_used:
results.append([id1, None] + row1[1:] + none_cols)
if join_type in ['FULL JOIN', 'RIGHT JOIN']:
none_cols = [None] * (len(table1[0]) - 1)
for id2, row2 in enumerate(table2):
if id2 not in table2_used:
results.append([None, id2] + none_cols + row2[1:])
if join_type == 'RIGHT JOIN':
results.sort(key=lambda x: x[1])
for r in results:
print(r[2:])
do_join('FULL JOIN')
do_join('LEFT JOIN')
do_join('RIGHT JOIN')
do_join('JOIN')
RESULTS
---------------------------------------------------------------
FULL JOIN
SELECT table1.col2, table2.col2 FROM table1 FULL JOIN table2 ON table1.col1=table2.col1;
['A', 'w']
['A', 'x']
['B', 'w']
['B', 'x']
['C', 'w']
['C', 'y']
['A', 'w']
['A', 'y']
['D', 'z']
['E', None]
['E', None]
[None, 's']
[None, 's']

---------------------------------------------------------------
LEFT JOIN
SELECT table1.col2, table2.col2 FROM table1 LEFT JOIN table2 ON table1.col1=table2.col1;
['A', 'w']
['A', 'x']
['B', 'w']
['B', 'x']
['C', 'w']
['C', 'y']
['A', 'w']
['A', 'y']
['D', 'z']
['E', None]
['E', None]

---------------------------------------------------------------
RIGHT JOIN
SELECT table1.col2, table2.col2 FROM table1 RIGHT JOIN table2 ON table1.col1=table2.col1;
['A', 'w']
['B', 'w']
['A', 'x']
['B', 'x']
['C', 'w']
['A', 'w']
['C', 'y']
['A', 'y']
['D', 'z']
[None, 's']
[None, 's']

---------------------------------------------------------------
JOIN
SELECT table1.col2, table2.col2 FROM table1 JOIN table2 ON table1.col1=table2.col1;
['A', 'w']
['A', 'x']
['B', 'w']
['B', 'x']
['C', 'w']
['C', 'y']
['A', 'w']
['A', 'y']
['D', 'z']

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