MySQL | 如何对查询结果集进行排序

数据操作语言：结果集排序

• 如果没有设置，查询语句不会对结果集进行排序。也就是说，如果想让结果集按照某种顺序排列，就必须使用 ORDER BY 子句。

SELECT ...... FROM ...... ORDER BY 列名 [ASC | DESC];

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SELECT ename,sal FROM t_emp ORDER BY sal;

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SELECT empno,ename,sal,deptno FROM t_emp ORDER BY sal DESC;

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排序关键字

• ASC 代表升序（默认），DESC 代表降序
• 如果排序列是数字类型，数据库就按照数字大小排序，如果是日期类型就按日期大小排序，如果是字符串就按照字符集序号排序。

SELECT ename,sal FROM t_emp ORDER BY hiredate DESC;

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	SELECT empno,ename,hiredate,deptno FROM t_emp ORDER BY ename ASC;
	SELECT empno,ename,hiredate,deptno FROM t_emp ORDER BY hiredate DESC;

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排序字段内容相同的情况

• 默认情况下，如果两条数据排序字段内容相同，那么排序会是什么样子？

• 我们可以使用 ORDER BY 规定首要排序条件和次要排序条件。

  数据库会先按照首要排序条件排序，如果遇到首要排序内容相同的记录，那么就会启用次要排序条件接着排序。

SELECT empno,ename,sal,hiredate FROM t_emp ORDER BY sal DESC,hiredate ASC;

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	SELECT
	empno,ename,deptno,sal
	FROM t_emp
	ORDER BY deptno,sal DESC;

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	SELECT
	empno,ename,sal
	FROM t_emp ORDER BY sal DESC LIMIT 0,5;

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排序 + 分页

• ORDER BY 子句书写的时候放在 LIMIT 子句的前面

  FROM -> SELECT -> ORDER BY -> LIMIT