SQL GROUP BY 以及 HAVING
合计函数 (比如 SUM) 常常需要添加 GROUP BY 功能。
GROUP BY...
把 GROUP BY 加入 SQL 的原因是，合计函数（比如 SUM）在每次被调用后会返回所有列的合计，如果没有 GROUP BY，就无法计算每个单独列值组合的总和。
GROUP BY 的语法：
SELECT column,SUM(column) FROM table GROUP BY column
GROUP BY 实例
表 "Sales":
| Company |
Amount |
| W3School |
6500 |
| IBM |
5500 |
| W3School |
7300 |
SQL:
SELECT Company, SUM(Amount) FROM Sales
结果:
| Company |
SUM(Amount) |
| W3School |
19300 |
| IBM |
19300 |
| W3School |
19300 |
上面的代码是无效的，这是由于被返回的列没有进行部分合计。GROUP BY 子句能解决这个问题：
SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company
结果:
| Company |
SUM(Amount) |
| W3School |
13800 |
| IBM |
5500 |
HAVING...
把 HAVING 加入 SQL 的原因是，WHERE 无法应用于合计函数，而如果没有 HAVING，就无法测试结果条件。
HAVING 的语法:
SELECT column,SUM(column) FROM table
GROUP BY column
HAVING SUM(column) condition value
表 "Sales":
| Company |
Amount |
| W3School |
6500 |
| IBM |
5500 |
| W3School |
7300 |
SQL:
SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company
HAVING SUM(Amount)>10000
结果：
| Company |
SUM(Amount) |
| W3School |
13800 |