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joins.pgsql
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joins.pgsql
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-- There are different kinds of joins
-- Inner and outer
-- ANd outer is further split into
-- Left outer join or left join
-- Right outer join or right join
-- Full outer join or full join
-- Besides these, there's also natural joins and cross join
-- Natural join joins on common columns (Not recommended as this is ambigous)
-- Cross join that gives you every permutation (every row with every other row in the table)( Not recommended as this is expensive)
-- Inner Join
-- returns related records only
-- See the set relations for joins here 👇
-- https://commons.wikimedia.org/wiki/File:SQL_Joins.svg
-- INNER JOIN syntax
SELECT fields FROM
table_A table_A_alias
INNER JOIN
table_B table_B_alias
ON [condition]
-- inner keyword can be left out as this is the default join
-- example
SELECT title, body FROM
recipes recipe
INNER JOIN
recipes_photos photo
ON
photo.recipe_id = recipe.id;
-- outer left syntax
SELECT fields FROM
table_a table_a_alias
OUTER LEFT JOIN
table_b table_b_alias
ON [condition]
-- outer keyword can be left out as left join is always an outer join.
-- example
SELECT title, body FROM
recipes recipe
LEFT JOIN
recipes_photos photo
ON
photo.recipe_id = recipe.id;
-- outer right syntax
SELECT fields FROM
table_a table_a_alias
OUTER RIGHT JOIN
table_b table_b_alias
ON [condition]
-- outer keyword can be left out as right join is always an outer join.
-- example
SELECT title, body FROM
recipes recipe
RIGHT JOIN
recipes_photos photo
ON
photo.recipe_id = recipe.id;
-- outer full join syntax
SELECT fields FROM
table_a table_a_alias
OUTER FULL JOIN
table_b table_b_alias
ON [condition]
-- outer keyword can be left out as left join is always an outer join.
-- example
SELECT title, body FROM
recipes recipe
FULL JOIN
recipes_photos photo
ON
photo.recipe_id = recipe.id;
-- Natural join example
SELECT fields FROM
table_a table_a_alias
NATURAL JOIN
table_b table_b_alias
-- Natural join example
SELECT title, body FROM
recipes
NATURAL JOIN
recipes_photos
-- If there is no common columns, it becomes a cross join.
-- Cross join
SELECT fields FROM
table_a table_a_alias
CROSS JOIN
table_b table_b_alias
-- Cross join example
SELECT * FROM
ingredients
CROSS JOIN
recipes;