Aggregate function (commonly used)
| Function name | description |
|---|---|
| Conuţ () | Calculate |
| SUM() | Sum |
| AVG() | average value |
| MAX() | Max |
| MIN () | Minimum |
-- ================= 聚合函数 ===============
-- 都能统计表中的数据
SELECT COUNT(`subject`) FROM `subject` -- count(字段) 会忽略所有的 null 值
SELECT COUNT(*) FROM `subject` -- count(*) 不会忽略 null 值, 本质是计算行数
SELECT COUNT(1) FROM `subject` -- count(1) 不会忽略 null 值, 计算行数
SELECT SUM(`gradeid`) AS 总和 FROM `subject`
SELECT AVG(`gradeid`) AS 平均值 FROM `subject`
SELECT MAX(`gradeid`) AS 最大值 FROM `subject`
SELECT MIN(`gradeid`) AS 最小值 FROM `subject`