mysql 根据表中字段值的不同，查询不同的关联表，该如何做

[fewf44]

求高手帮忙
我要查询一张表：假如里面有2个字段 a 和b 。如果a =0 ，我就根据b这个字段的值查询所有和a有关联的关联表 如果a =1，我就查询所有和b有关联的关联表

[zbdzjx]

a=0的查询union all a=1的查询

[0808xyj]

 select A.*,isnull(B.val,C.val)
 from A
 left join B on A.keyid=B.keyid and a.a=0
 left join C on A.keyID=c.Keyid and a.a=1
 where a.a in(0,1)

[哈睿]

SELECT 
    CASE WHEN t1.type=1 THEN 
        (SELECT student_name FROM `学生表` t2 WHERE t2.student_id=t1.user_id)
    WHEN t1.type=2 THEN
        (SELECT teacher_name FROM `学生表` t2 WHERE t2.teacher_id=t1.user_id)
    ELSE END AS '姓名',
    CASE WHEN t1.type=1 THEN '学生'    WHEN t1.type=2 THEN '老师' ELSE END AS '对象类型',   
    t1.reason AS '请假原因',
    t1.
FROM `请假表` t1;

试过，可行。