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SQL Joins

The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables.

Tables in a database are often related to each other with keys.

A primary key is a column (or a combination of columns) with a unique value for each row. Each primary key value must be unique within the table. The purpose is to bind data together, across tables, without repeating all of the data in every table


FULL OUTER JOIN

A JOIN is made matching a column on a table to a column on the other table. After a FULL OUTER JOIN, for a given value (red), for a given row with this value on one table ([ red | 9999 ]), one row is created for each row that matches on the other table ([ red | OOOOOO ] and [ red | LLLLLL ]). If a value exists in only one table, then a row is created and is completed with NULL columns.
FROM table_1 FULL OUTER JOIN table_2 ON
 table_1.common_value = table_2.common_value


common_valuespecific_value_1specific_value_2
red9999OOOOOO
red9999LLLLLL
grey6666NULL
white0000NULL
purple7777NULL
purple2222NULL
black8888FFFFFF
greenNULLHHHHHH
yellowNULLPPPPPP
blueNULLRRRRRR


RIGHT OUTER JOIN 

The RIGHT OUTER JOIN is like the FULL OUTER JOIN but it doesn't create row for values that don't exist on the left table.
FROM table_1 RIGHT OUTER JOIN table_2 ON
 table_1.common_value = table_2.common_value


common_valuespecific_value_1specific_value_2
red9999OOOOOO
red9999LLLLLL
black8888FFFFFF
greenNULLHHHHHH
yellowNULLPPPPPP
blueNULLRRRRRR

LEFT OUTER JOIN 

The LEFT OUTER JOIN is like the FULL OUTER JOIN but it doesn't create row for values that don't exist on the right table.
FROM table_1 LEFT OUTER JOIN table_2 ON
 table_1.common_value = table_2.common_value
common_valuespecific_value_1specific_value_2
red9999OOOOOO
red9999LLLLLL
grey6666NULL
white0000NULL
purple7777NULL
purple2222NULL
black8888FFFFFF

INNER JOIN 

The INNER JOIN is like the FULL OUTER JOIN but it creates row only for values that exist on both the left table and the right table.
FROM table_1 INNER JOIN table_2 ON
 table_1.common_value = table_2.common_value
common_valuespecific_value_1specific_value_2
red9999OOOOOO
red9999LLLLLL
black8888FFFFFF


Originally posted in May 2013
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