在 postgresql 中使用 SELECT EXISTS 时如何返回多个值

Question

我有以下 SQL 查询：

SELECT EXISTS (SELECT r.id FROM Rules r INNER JOIN rule_t c on c.id=r.rule_t.id 
INNER JOIN user u on u.id = r.user_id
WHERE u.fmnum='2813'
AND c.name='default') ::int

有没有办法可以修改它，以便我得到两个值，来自 EXISTS 方法的 INT 和 r.id？

我知道我可以更改查询以便删除 EXISTS 方法...如果子选择返回任何内容，那么我知道记录存在...但我只是想知道是否可以这样做以上。

谢谢。

编辑 1

我正在 pgadmin3 的新查询窗口中测试以下代码...

SELECT *
FROM  (
   SELECT TRUE, r.id
   FROM   rules  r
   JOIN   rule_t c on c.id = r.rule_t.id 
   JOIN   user   u on u.id = r.user_id
   WHERE  u.fmnum = '2813'
   AND    c.name = 'default'
   );

但我收到以下错误：

错误：FROM 中的子查询必须有别名 LINE 2: ( ^ 提示：例如，FROM (SELECT ...) [AS] foo。

编辑 2

SELECT *
FROM  (
   SELECT TRUE, r.id
   FROM   rules  r
   JOIN   rule_t c on c.id = r.rule_t.id 
   JOIN   user   u on u.id = r.user_id
   WHERE  u.fmnum = '2813'
   AND    c.name = 'default'
   ) AS x;

Answer 1

SELECT 1 AS does_exist, r.id
FROM   rules  r
JOIN   rule_t c on c.id = r.rule_t.id 
JOIN   user   u on u.id = r.user_id
WHERE  u.fmnum = '2813'
AND    c.name = 'default'
LIMIT  1;  -- may or may not be needed.

这符合您的要求：您得到两列。但是如果什么也没找到，你会得到no row。

如果你想要一行，即使没有找到，你也需要一个子查询：

SELECT sub.t_id IS NOT NULL AS does_exist, sub.id
FROM  (SELECT 1) x  -- dummy to guarantee 1 row
LEFT JOIN (         -- LEFT JOIN is crucial
    SELECT r.id
    FROM   rules  r
    JOIN   rule_t c on c.id = r.rule_t.id 
    JOIN   user   u on u.id = r.user_id
    WHERE  u.fmnum = '2813'
    AND    c.name = 'default'
    LIMIT  1        -- may or may not be needed.
    ) ON TRUE;      -- join condition is always true

或者，更简单/更快：

SELECT 1 AS does_exist, r.id
FROM   rules  r
JOIN   rule_t c on c.id = r.rule_t.id 
JOIN   user   u on u.id = r.user_id
WHERE  u.fmnum = '2813'
AND    c.name = 'default'

UNION  ALL
SELECT 0, NULL
LIMIT  1;