如何组合多个查询？

Question

我有一些类似的问题：

select sysdate from dual;
select id, count(*) as A from TABLE1 group by id;
select id, count(*) as B from TABLE2 group by id;
select id, count(*) as C from TABLE3 group by id;
select id, count(*) as D from TABLE4 group by id;

如何组合这5个查询：

谢谢。

Answer 1

您可以以有意义的方式将四个表联合在一起，然后进行条件聚合：

SELECT
    SYSDATE,
    id,
    SUM(CASE WHEN label = 'A' THEN 1 ELSE 0 END) AS A,
    SUM(CASE WHEN label = 'B' THEN 1 ELSE 0 END) AS B
    SUM(CASE WHEN label = 'C' THEN 1 ELSE 0 END) AS C,
    SUM(CASE WHEN label = 'D' THEN 1 ELSE 0 END) AS D
FROM
(
    SELECT id, 'A' AS label
    FROM TABLE1
    UNION ALL
    SELECT id, 'B'
    FROM TABLE2
    UNION ALL
    SELECT id, 'C'
    FROM TABLE3
    UNION ALL
    SELECT id, 'D'
    FROM TABLE4
) t
GROUP BY id;

Answer 2

使用union关键字。

来使用此功能