碎片时间杂学 6.14 数据库基础补漏

别名：

select abc AS a，def AS d from paramtable where a = ‘test’；

查询常数：

SELECT '王者荣耀' as platform, name FROM heros

去除重复行：

SELECT DISTINCT attack_range FROM heros

排序检索数据：

SELECT name, hp_max FROM heros ORDER BY hp_max DESC

约束返回结果的数量：

SELECT name, hp_max FROM heros ORDER BY hp_max DESC LIMIT 5

SELECT 的执行顺序：

1. 关键词顺序（不能颠倒）

SELECT ... FROM ... WHERE ... GROUP BY ... HAVING ... ORDER BY ...

2. 执行顺序

FROM > WHERE > GROUP BY > HAVING > SELECT的字段 > DISTINCT > ORDER BY > LIMIT

举例：

SELECT DISTINCT player_id, player_name, count(*) as num #顺序5

FROM player JOIN team ON player.team_id = team.team_id #顺序1

WHERE height > 1.80 #顺序2

GROUP BY player.team_id #顺序3

HAVING num > 2 #顺序4

ORDER BY num DESC #顺序6

LIMIT 2 #顺序7