mysql 这个怎么写

Question

数据库1 2 3 4
以1的某个字段a判断，是b则select查询获取2中字段C，是c则查询获取3中字段d，是d则查询获取4中字段e，这个该怎么写，谢谢

Answer 1

SELECT
    CASE a
        WHEN 'b' THEN (
            SELECT C
            FROM database2.table2
            WHERE condition
        )
        WHEN 'c' THEN (
            SELECT d
            FROM database3.table3
            WHERE condition
        )
        WHEN 'd' THEN (
            SELECT e
            FROM database4.table4
            WHERE condition
        )
        ELSE NULL
    END AS result
FROM database1.table1
WHERE condition;