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Suppose I have table A with a field that can be either 1 or 2...
How do I select such that for each row in table A, if the field is 1, join the select with table B and if the field is 2, join the select with table C?
632
You join two tables by creating a relationship in the WHERE clause between at least one column from one table and at least one column from another. The join creates a temporary composite table where each pair of rows (one from each table) that satisfies the join condition is linked to form a single row.
A conditional column join is a fancy way to let us join to a single column and to two (or more) columns in a single query. We can accomplish this by using a case statement in the on clause of our join. A case statement allows us to test multiple conditions (like an if/else if/else) to produce a single value.
The join is done by the JOIN operator. In the FROM clause, the name of the first table ( product ) is followed by a JOIN keyword then by the name of the second table ( category ). This is then followed by the keyword ON and by the condition for joining the rows from the different tables.
SQL JOIN. A JOIN clause is used to combine rows from two or more tables, based on a related column between them.
(
SELECT MyField1, MyField2 FROM A
INNER JOIN B ON A.Id = B.Id
AND A.MyField = 1
)
UNION
(
SELECT MyField1, MyField2 FROM A
INNER JOIN C ON A.Id = C.Id
AND A.MyField = 2
)
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