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() 函数时会正常工作。但是，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。
我创建了一张有四列的表老师，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)
所以它会给我返回同步状态的完美总和。但如果我将使用带条件的 SUM() 则返回记录数。
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)