Question

以下是在mysql 5.6服务器上运行的名为pages的表的模式

+---------------+------------------+------+-----+---------+----------------+
| Field         | Type             | Null | Key | Default | Extra          |
+---------------+------------------+------+-----+---------+----------------+
| id            | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| city          | varchar(255)     | YES  |     | NULL    |                |
| email         | varchar(255)     | YES  |     | NULL    |                |
+---------------+------------------+------+-----+---------+----------------+

我想将以下2个sql查询组合成单个sql查询，这可能吗？我尝试使用嵌套查询，但没有得到理想的结果。

select count(*) from pages where email != "";
select count(*) from pages where email = "";

由于

Answer 1

在MySQL中，你可以非常干净地写出来：

select sum(email <> "") not_blank, sum(email = "") blank from pages;

它使用了以下事实：true计算结果为1，而在MySQL中，false计算结果为0。

在其他DBMS中，等效查询将是：

select sum(case when email <> "" then 1 else 0 end) not_blank,
    sum(case when email = "" then 1 else 0 end) blank
from pages;

如果您想使用COUNT：

select count(case when email <> "" then 1 end) not_blank,
    count(case when email = "" then 1 end) blank
from pages;

Answer 2

您甚至可以使用CASE表达式。

<强>查询

select 
  sum(case when trim(`email`) = "" or `email` is null then 1 else 0 end) `is_null`,
  sum(case when trim(`email`) <> "" or `email` is not null then 1 else 0 end) `is_not_null`
from `pages`;

Answer 3

SELECT SUM(NOT_BLANK) NOT_BLANK,SUM(BLANK) BLANK FROM
(
select count(*) as NOT_BLANK,0 as BLANK from pages where email != "";
UNION ALL
select 0 as NOT_BLANK,count(*) as BLANK from pages where email = "";
)ZZ

请尝试上面的代码，希望这会有所帮助。

单个sql查询中相同字段上的相等，不相等的条件

3 个答案: