mysql查询的语法
单表查询语法
SELECT DISTINCT 字段1,字段2... FROM 表名 WHERE 条件 GROUP BY field HAVING 筛选 ORDER BY field LIMIT 限制条数
关键字执行的优先级
from where group by select having distinct order by limit
1.找到表:from
2.拿着where指定的约束条件,去文件/表中取出一条条记录
3.将取出的一条条记录进行分组group by,如果没有group by,则整体作为一组
4.执行select(去重)
5.将分组的结果进行having过滤
6.将结果按条件排序:order by
7.限制结果的显示条数
where约束
where字句中可以使用:
1. 比较运算符:> < >= <= <> != 2. between 80 and 100 值在80到100之间 3. in(80,90,100) 值是80或90或100 4. like 'e%' 通配符可以是%或_, %表示任意多字符 _表示一个字符 5. 逻辑运算符:在多个条件直接可以使用逻辑运算符 and or not
group by
单独使用GROUP BY关键字分组 SELECT post FROM employee GROUP BY post; 注意:我们按照post字段分组,那么select查询的字段只能是post,想要获取组内的其他相关信息,需要借助函数 GROUP BY关键字和GROUP_CONCAT()函数一起使用 SELECT post,GROUP_CONCAT(emp_name) FROM employee GROUP BY post;#按照岗位分组,并查看组内成员名 SELECT post,GROUP_CONCAT(emp_name) as emp_members FROM employee GROUP BY post; GROUP BY与聚合函数一起使用 select post,count(id) as count from employee group by post;#按照岗位分组,并查看每个组有多少人
聚合函数
#强调:聚合函数聚合的是组的内容,若是没有分组,则默认一组 示例: SELECT COUNT(*) FROM employee; SELECT COUNT(*) FROM employee WHERE depart_id=1; SELECT MAX(salary) FROM employee; SELECT MIN(salary) FROM employee; SELECT AVG(salary) FROM employee; SELECT SUM(salary) FROM employee; SELECT SUM(salary) FROM employee WHERE depart_id=3;
HAVING过滤
HAVING与WHERE不一样的地方在于!!!!!!
ORDER BY 查询排序
按单列排序 SELECT * FROM employee ORDER BY salary; SELECT * FROM employee ORDER BY salary ASC; SELECT * FROM employee ORDER BY salary DESC; 按多列排序:先按照age排序,如果年纪相同,则按照薪资排序 SELECT * from employee ORDER BY age, salary DESC;
LIMIT 限制查询的记录数
示例: SELECT * FROM employee ORDER BY salary DESC LIMIT 3; #默认初始位置为0 SELECT * FROM employee ORDER BY salary DESC LIMIT 0,5; #从第0开始,即先查询出第一条,然后包含这一条在内往后查5条 SELECT * FROM employee ORDER BY salary DESC LIMIT 5,5; #从第5开始,即先查询出第6条,然后包含这一条在内往后查5条
使用正则表达式查询
SELECT * FROM employee WHERE emp_name REGEXP '^ale'; SELECT * FROM employee WHERE emp_name REGEXP 'on$'; SELECT * FROM employee WHERE emp_name REGEXP 'm{2}'; 小结:对字符串匹配的方式 WHERE emp_name = 'egon'; WHERE emp_name LIKE 'yua%'; WHERE emp_name REGEXP 'on$';
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