Question

如何从SELECT语句的子查询中获得非空结果？

SELECT a, b, c,
       (SELECT d 
        FROM table2
        WHERE ...) as d
FROM table 1
WHERE ...

我只想在所有值（a，b，c，d）不为Null时获得结果。在主WHERE子句中也使用相同的子查询，但使用EXISTS并不是一种奇怪/低效率的方法吗？

Answer 1

最简单的方法是将原始查询放在子查询中，然后可以检查子查询返回的整行是否为NULL：

SELECT *
FROM (
    SELECT a, b, c,
           (SELECT d 
            FROM table2
            WHERE ...)
    FROM table 1
    WHERE ...
) AS sub
WHERE sub IS NOT NULL

sub是子查询返回的(a,b,c,d)行。

Answer 2

SELECT 
 t1.a,
 t1.b,
 t1.c, 
 t2.d
FROM table1 t1
left join table2 as t2 on t2.ID = t1.ID
WHERE t1.a is not null and t1.b is not null and t1.c is not null and t2.d is not null

Answer 3

您可以使用子查询：

select a, b, c, d
from (SELECT a, b, c,
             (SELECT d 
              FROM table2
              WHERE ...) as d
      FROM table 1
      WHERE ... and
            a is not null and b is not null and c is not null
     ) x
where d is not null;

不过，您很可能可以使用JOIN：

SELECT a, b, c, x.d 
FROM table 1 JOIN
      (SELECT d 
        FROM table2
        WHERE ...
     ) x
WHERE ... and
      a is not null and b is not null and c is not null and d is not null;

检查SELECT中子查询的结果是否存在

3 个答案: