mysql求和函数问题

我对mysql中的SUM（）函数有一些疑问。当我在没有条件的情况下使用SUM（）函数时，它可以正常工作。但如果我将SUM（）函数与条件一起使用，它将返回与条件匹配的记录数。给我解释一下

我创建了一个包含四列的表格教师，如下所示

mysql> desc Teacher;


+--------------+-------------+------+-----+-------------------+
| Field        | Type        | Null | Key | Default           |
+--------------+-------------+------+-----+-------------------+
| TeacherId    | varchar(36) | NO   | PRI | NULL              |
| FirstName    | varchar(50) | NO   |     | NULL              |
| LastName     | varchar(50) | NO   |     | NULL              |
| SyncStatus   | int(11)     | NO   |     | NULL              |
+--------------+-------------+------+-----+-------------------+

它包含以下记录。

mysql> select * from Teacher;

+-----------+-----------+-----------+------------+
| TeacherId | FirstName | LastName  | SyncStatus |
+-----------+-----------+-----------+------------+
| 001       | Sagar     | Kapadia   |          1 |
| 002       | Vishal    | Chaudhari |          1 |
| 003       | Milan     | Panchal   |          1 |
| 004       | Jugal     | Desai     |          1 |
| 005       | Baburao   | Apte      |          1 |
| 006       | Jaya      | Bacchan   |          0 |
| 007       | Jaya      | Bacchan   |          2 |
| 008       | Jaya      | Bacchan   |          2 |
+-----------+-----------+-----------+------------+
8 rows in set (0.00 sec)




mysql> select TeacherId,FirstName,LastName,Concat(FirstName,' ',LastName) as FullName,SUM(SyncStatus) from Teacher having FullName like '_____ %';
+-----------+-----------+----------+---------------+-----------------+
| TeacherId | FIrstName | LastName | FullName      | sum(SyncStatus) |
+-----------+-----------+----------+---------------+-----------------+
| 001       | Sagar     | Kapadia  | Sagar Kapadia |               9 |
+-----------+-----------+----------+---------------+-----------------+
1 row in set (0.00 sec)

mysql> select TeacherId,FIrstName,LastName,Concat(FirstName,' ',LastName) as FullName,sum(SyncStatus = 0) from Teacher having FullName like '_____ %';
+-----------+-----------+----------+---------------+---------------------+
| TeacherId | FIrstName | LastName | FullName      | sum(SyncStatus = 0) |
+-----------+-----------+----------+---------------+---------------------+
| 001       | Sagar     | Kapadia  | Sagar Kapadia |                   1 |
+-----------+-----------+----------+---------------+---------------------+
1 row in set (0.00 sec)



mysql> select TeacherId,FIrstName,LastName,Concat(FirstName,' ',LastName) as FullName,sum(SyncStatus = 1) from Teacher having FullName like '_____ %';
+-----------+-----------+----------+---------------+---------------------+
| TeacherId | FIrstName | LastName | FullName      | sum(SyncStatus = 1) |
+-----------+-----------+----------+---------------+---------------------+
| 001       | Sagar     | Kapadia  | Sagar Kapadia |                   5 |
+-----------+-----------+----------+---------------+---------------------+
1 row in set (0.00 sec)



mysql> select TeacherId,FIrstName,LastName,Concat(FirstName,' ',LastName) as FullName,sum(SyncStatus = 2) from Teacher having FullName like '_____ %';
+-----------+-----------+----------+---------------+---------------------+
| TeacherId | FIrstName | LastName | FullName      | sum(SyncStatus = 2) |
+-----------+-----------+----------+---------------+---------------------+
| 001       | Sagar     | Kapadia  | Sagar Kapadia |                   2 |
+-----------+-----------+----------+---------------+---------------------+
1 row in set (0.00 sec)

因此它将在同步状态下返回完美的总和。但如果我将SUM（）与条件一起使用，则返回记录数。

mysql> select * from Teacher;

+-----------+-----------+-----------+------------+
| TeacherId | FirstName | LastName  | SyncStatus |
+-----------+-----------+-----------+------------+
| 001       | Sagar     | Kapadia   |          1 |
| 002       | Vishal    | Chaudhari |          1 |
| 003       | Milan     | Panchal   |          1 |
| 004       | Jugal     | Desai     |          1 |
| 005       | Baburao   | Apte      |          1 |
| 006       | Jaya      | Bacchan   |          0 |
| 007       | Jaya      | Bacchan   |          2 |
| 008       | Jaya      | Bacchan   |          2 |
+-----------+-----------+-----------+------------+
8 rows in set (0.00 sec)




mysql> select TeacherId,FirstName,LastName,Concat(FirstName,' ',LastName) as FullName,SUM(SyncStatus) from Teacher having FullName like '_____ %';
+-----------+-----------+----------+---------------+-----------------+
| TeacherId | FIrstName | LastName | FullName      | sum(SyncStatus) |
+-----------+-----------+----------+---------------+-----------------+
| 001       | Sagar     | Kapadia  | Sagar Kapadia |               9 |
+-----------+-----------+----------+---------------+-----------------+
1 row in set (0.00 sec)

mysql> select TeacherId,FIrstName,LastName,Concat(FirstName,' ',LastName) as FullName,sum(SyncStatus = 0) from Teacher having FullName like '_____ %';
+-----------+-----------+----------+---------------+---------------------+
| TeacherId | FIrstName | LastName | FullName      | sum(SyncStatus = 0) |
+-----------+-----------+----------+---------------+---------------------+
| 001       | Sagar     | Kapadia  | Sagar Kapadia |                   1 |
+-----------+-----------+----------+---------------+---------------------+
1 row in set (0.00 sec)



mysql> select TeacherId,FIrstName,LastName,Concat(FirstName,' ',LastName) as FullName,sum(SyncStatus = 1) from Teacher having FullName like '_____ %';
+-----------+-----------+----------+---------------+---------------------+
| TeacherId | FIrstName | LastName | FullName      | sum(SyncStatus = 1) |
+-----------+-----------+----------+---------------+---------------------+
| 001       | Sagar     | Kapadia  | Sagar Kapadia |                   5 |
+-----------+-----------+----------+---------------+---------------------+
1 row in set (0.00 sec)



mysql> select TeacherId,FIrstName,LastName,Concat(FirstName,' ',LastName) as FullName,sum(SyncStatus = 2) from Teacher having FullName like '_____ %';
+-----------+-----------+----------+---------------+---------------------+
| TeacherId | FIrstName | LastName | FullName      | sum(SyncStatus = 2) |
+-----------+-----------+----------+---------------+---------------------+
| 001       | Sagar     | Kapadia  | Sagar Kapadia |                   2 |
+-----------+-----------+----------+---------------+---------------------+
1 row in set (0.00 sec)

---

你说得对。此函数

sum（SyncStatus=2）

将返回行数，因为这就是它的执行方式

---

在每一行中，

SyncStatus=2

将测试

SyncStatus是否等于2

，并返回

1（真）或0（假）

，因此您基本上得到了真值的总和（1）

谢谢@Ankur。我现在了解了SUM（）函数将如何执行。再次感谢你……刚刚遇到了这种事情——它看起来像是在做我想做的事情（使用

SUM（confirmed=1）

和

groupby

）返回一个月内完成操作的次数，但我不确定它是否应该这样做——现在我知道了：）