“ FROM”语句中的“ IF”-根据列值选择表值

Question

SELECT CI FROM users WHERE something;

IF users.CI='pc' THEN
    SELECT name FROM table1 WHERE something;
ELSE IF users.CI='ph' THEN
    SELECT name FROM table2 WHERE something;
END IF

I know that doesn't work, but is an example to understand.

It's possible all of this in one query ?

Answer 1

SET @var = (SELECT CI FROM users WHERE something);

SELECT
    CASE @var
        WHEN 'pc' THEN
            (SELECT name FROM table1 WHERE something)
        WHEN 'ph' THEN
            (SELECT name FROM table2 WHERE something)
    END;

Answer 2

You cannot use IF ... THEN in a SQL query.

The solution is to left outer join both tables, letting the optimizer choose which table will be used by specifying so in the ON clauses.

You didn't explain what "something" was, so in this example, I am going to assume that "users" has an "id" column and that table1 and table2 have a "user_id" column, and that you are interested in the user with id = 1.

SELECT COALESCE(t1.name,t2.name) AS name
  FROM users u
  LEFT JOIN table1 t1 ON u.CI = 'pc' AND t1.user_id = u.id
  LEFT JOIN table2 t2 ON u.CI = 'ph' AND t2.user_id = u.id
 WHERE u.id = 1;