MySql案例什么时候需要在哪里呢？

我写了这个查询：

SELECT cola, colb, colc, CASE colb
WHEN '6kHcnevOJOSU' THEN 0
WHEN 'g45ujP0td6nw' THEN 1
WHEN 'v83f15lALyFs' THEN 2
END AS sor FROM mytable

“sor”列返回正确，问题是查询返回所有行，而不仅仅是其中一个案例与colb匹配的行！ 如何使其仅返回匹配的行

我需要添加一个WHERE？但我总是看到这种语法没有它。谢谢

模式：

drop table if exists mytable;
create table mytable
(   id int auto_increment primary key,
    cola int not null,
    colb varchar(20) not null,
    colc int not null
);
insert mytable (cola,colb,colc) values
(9,'6kHcnevOJOSU',1),
(3,'v83f15lALyFs',1),
(9,'frog',1),
(3,'g45ujP0td6nw',1);

备选案文1：

SELECT cola, colb, colc, CASE colb
WHEN '6kHcnevOJOSU' THEN 0
WHEN 'g45ujP0td6nw' THEN 1
WHEN 'v83f15lALyFs' THEN 2
END AS sor FROM mytable
where colb in ('6kHcnevOJOSU','g45ujP0td6nw','v83f15lALyFs');

备选案文2：

select cola,colb,colc,sor
from
(   SELECT cola, colb, colc, CASE colb
    WHEN '6kHcnevOJOSU' THEN 0
    WHEN 'g45ujP0td6nw' THEN 1
    WHEN 'v83f15lALyFs' THEN 2
    END AS sor FROM mytable
) xDerived
where sor is not null;

结果:

+------+--------------+------+------+
| cola | colb         | colc | sor  |
+------+--------------+------+------+
|    9 | 6kHcnevOJOSU |    1 |    0 |
|    3 | v83f15lALyFs |    1 |    2 |
|    3 | g45ujP0td6nw |    1 |    1 |
+------+--------------+------+------+

在选项1中，

sor

在

where

子句中尚不可用

---

在选项2中，派生表

xDerived

清除外部包装器对

sor

的使用

只需添加一个

where

子句：

SELECT cola, colb, colc,
       (CASE colb WHEN '6kHcnevOJOSU' THEN 0 
                  WHEN 'g45ujP0td6nw' THEN 1
                  WHEN 'v83f15lALyFs' THEN 2
        END) as sor
FROM mytable t
WHERE colc IN (0, 1, 2);

SELECT cola, colb, colc,
       (CASE colb WHEN '6kHcnevOJOSU' THEN 0 
                  WHEN 'g45ujP0td6nw' THEN 1
                  WHEN 'v83f15lALyFs' THEN 2
        END) as sor
FROM mytable t
HAVING sor IS NOT NULL;

或者，您可以使用

having

子句：

SELECT cola, colb, colc,
       (CASE colb WHEN '6kHcnevOJOSU' THEN 0 
                  WHEN 'g45ujP0td6nw' THEN 1
                  WHEN 'v83f15lALyFs' THEN 2
        END) as sor
FROM mytable t
WHERE colc IN (0, 1, 2);

SELECT cola, colb, colc,
       (CASE colb WHEN '6kHcnevOJOSU' THEN 0 
                  WHEN 'g45ujP0td6nw' THEN 1
                  WHEN 'v83f15lALyFs' THEN 2
        END) as sor
FROM mytable t
HAVING sor IS NOT NULL;

---

是的，如果要过滤结果，需要一个

where

子句。对于不匹配的记录，当前查询将返回最后一列的

null

值。第二列似乎很完美！我看到你被父母取代了，有什么区别吗？@Jackt。实际上，我通常将

大小写

放在括号中（更容易阅读），并且总是将

用作列别名。