Question

我想知道是否可以在CAS子句中放入CASE语句......在MySQL中？类似的东西：

SELECT id1 FROM ref_table AS rt WHERE
EXISTS 
   (SELECT rt.id FROM 
      (CASE rt.table
         WHEN 'tbl1' THEN tbl_1
         WHEN 'tbl2' THEN tbl_2
         WHEN 'tbl3' THEN tbl_3
      END) AS tbl
   WHERE tbl.id2 = rt.id_table)

Answer 1

CASE表达式的结果不能是表，所以不，你不能使用这种语法。您可以使用OR（或CASE）和3个EXISTS子查询重写查询：

SELECT id1 FROM ref_table AS rt WHERE
CASE rt.table
    WHEN 'tbl1' THEN EXISTS (SELECT 1 FROM tbl_1 t WHERE t.id2 = rt.id_table)
    WHEN 'tbl2' THEN EXISTS (SELECT 1 FROM tbl_2 t WHERE t.id2 = rt.id_table)
    WHEN 'tbl3' THEN EXISTS (SELECT 1 FROM tbl_3 t WHERE t.id2 = rt.id_table)
END ;

更具可读性，OR：

SELECT id1 FROM ref_table AS rt WHERE
   rt.table = 'tbl1' AND EXISTS (SELECT 1 FROM tbl_1 t WHERE t.id2 = rt.id_table)
OR rt.table = 'tbl2' AND EXISTS (SELECT 1 FROM tbl_2 t WHERE t.id2 = rt.id_table)
OR rt.table = 'tbl3' AND EXISTS (SELECT 1 FROM tbl_3 t WHERE t.id2 = rt.id_table)
;

MySQL：FROM子句中的CASE语句

1 个答案: