Oracle 子查询执行

SELECT CASE a.x
         WHEN 'b' THEN (SELECT b.y FROM b WHERE b.id = a.b_id)
         WHEN 'c' THEN (SELECT c.y FROM c WHERE c.id = a.c_id)
         WHEN 'd' THEN (SELECT d.y FROM d WHERE d.id = a.d_id)
       END
  FROM a

SQL> create table a ( x char(1));

Table created.

SQL> insert into a values ('b');

1 row created.

SQL>
SQL> create table b ( y int );

Table created.

SQL> insert into b values (1);

1 row created.

SQL>
SQL> SELECT CASE a.x
  2           WHEN 'b' THEN (SELECT b.y FROM b )
  3           WHEN 'c' THEN (SELECT y/0 FROM b )
  4           WHEN 'd' THEN (SELECT y/0 FROM b )
  5         END
  6    FROM a;

CASEA.XWHEN'B'THEN(SELECTB.YFROMB)WHEN'C'THEN(SELECTY/0FROMB)WHEN'D'THEN(SELECTY/0FROMB)END
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                                                                                          1

1 row selected.