mysql怎么进⾏单表查询_MySQL之单表查询⼀、单表查询的语法
SELECT 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
⼆、关键字的执⾏顺序
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.将分组的结果进⾏having过滤
5.执⾏lect
6.去重
7.将结果按条件排序：order by
8.限制结果的显⽰条数
SELECT 字段1，字段2... ⑤
FROM 表名 ①
WHERE 条件 ②
GROUP BY field ③
HAVING 筛选 ④
ORDER BY field ⑥
LIMIT 限制条数 ⑦
三、简单查询
#简单查询
SELECT id,name,x,age,hire_date,post,post_comment,salary,office,depart_id
FROM employee;
SELECT * FROM employee;
SELECT name,salary FROM employee;
#避免重复DISTINCT
SELECT DISTINCT post FROM employee;
#通过四则运算查询
SELECT name, salary*12 FROM employee;
SELECT name, salary*12 AS Annual_salary FROM employee; SELECT name, salary*12 Annual_salary FROM employee;
#定义显⽰格式
CONCAT() 函数⽤于连接字符串
SELECT CONCAT('姓名: ',name,' 年薪: ', salary*12) AS Annual_salary FROM employee;
CONCAT_WS() 第⼀个参数为分隔符
SELECT CONCAT_WS(':',name,salary*12) AS Annual_salary
FROM employee;
结合CASE语句：
SELECT
(
CASE
WHEN NAME = 'egon' THEN
NAME
WHEN NAME = 'alex' THEN
CONCAT(name,'_BIGSB')
ELSE
concat(NAME, 'SB')
END
) as new_name
FROM
emp;
四、where约束
where字句中可以使⽤：
1. ⽐较运算符：> < >= <= <> !=
2. between 80 and 100 值在10到20之间
3. in(80,90,100) 值是10或20或30
4. like 'egon%' pattern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
#1：单条件查询
SELECT name FROM employee
WHERE post='sale';
#2：多条件查询
SELECT name,salary FROM employee
WHERE post='teacher' AND salary>10000;
#3：关键字BETWEEN AND
SELECT name,salary FROM employee
WHERE salary BETWEEN 10000 AND 20000;
SELECT name,salary FROM employee
WHERE salary NOT BETWEEN 10000 AND 20000;
#4：关键字IS NULL(判断某个字段是否为NULL不能⽤等号，需要⽤IS)
SELECT name,post_comment FROM employee
WHERE post_comment IS NULL;
SELECT name,post_comment FROM employee
WHERE post_comment IS NOT NULL;
SELECT name,post_comment FROM employee
WHERE post_comment=''; 注意''是空字符串，不是null
ps：
执⾏
update employee t post_comment='' where id=2;
再⽤上条查看，就会有结果了
#5：关键字IN集合查询
SELECT name,salary FROM employee
WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ; SELECT name,salary FROM employee
WHERE salary IN (3000,3500,4000,9000) ;
SELECT name,salary FROM employee
WHERE salary NOT IN (3000,3500,4000,9000) ;
#6：关键字LIKE模糊查询
通配符’%’
SELECT * FROM employee
WHERE name LIKE 'eg%';
通配符’_’
SELECT * FROM employee
WHERE name LIKE 'al__';
五 分组查询:GROUP BY
1 什么是分组？为什么要分组？
#1、⾸先明确⼀点：分组发⽣在where之后，即分组是基于where之后得到的记录⽽进⾏的
#2、分组指的是：将所有记录按照某个相同字段进⾏归类，⽐如针对员⼯信息表的职位分组，或者按照性别进⾏分组等
#3、为何要分组呢？
取每个部门的最⾼⼯资
取每个部门的员⼯数
取男⼈数和⼥⼈数
⼩窍门：‘每’这个字后⾯的字段，就是我们分组的依据#4、⼤前提：
可以按照任意字段分组，但是分组完毕后，⽐如group by post，只能查看post字段，如果想查看组内信息，需要借助于聚合函数2 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lect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(name) FROM employee GROUP BY post;#按照岗位分组，并查看组内成员名
SELECT post,GROUP_CONCAT(name) as emp_members FROM employee GROUP BY post;
GROUP BY与聚合函数⼀起使⽤
lect post,count(id) as countfrom employee group by post;#按照岗位分组，并查看每个组有多少⼈
强调：
如果我们⽤unique的字段作为分组的依据，则每⼀条记录⾃成⼀组，这种分组没有意义
多条记录之间的某个字段值相同，该字段通常⽤来作为分组的依据
3 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;
4 HAVING过滤
HAVING与WHERE不⼀样的地⽅在于
#执⾏优先级从⾼到低：where > group by > having#1. Where 发⽣在分组group by之前，因⽽Where中可以有任意字段，但是绝对不能使⽤聚合函数。
#2. Having发⽣在分组group by之后，因⽽Having中可以使⽤分组的字段，⽆法直接取到其他字段，可以使⽤聚合函数
mysql>lect @@sql_mode;+--------------------+
| @@sql_mode |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
1 row in t (0.00c)
mysql> lect * from emp where salary > 100000;+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | name | x | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| 2 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
1 row in t (0.00c)
mysql> lect * from emp having salary > 100000;
ERROR1463 (42000): Non-grouping field 'salary' is ud inHAVING clau
mysql> lect post,group_concat(name) from emp group by post having salary > 10000;#错误，分组后⽆法直接取到salary字段
ERROR 1054 (42S22): Unknown column 'salary' in 'having clau'mysql> lect post,group_concat(
name) from emp group by post having avg(salary) > 10000;+-----------+-------------------------------------------------------+
| post | group_concat(name) |
+-----------+-------------------------------------------------------+
| operation | 程咬铁，程咬铜，程咬银，程咬⾦，张野 |
| teacher | 成龙,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |
+-----------+-------------------------------------------------------+
2 rows in t (0.00 c)
验证
六 查询排序:ORDER BY
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
