标签:语句 salary employees 查询 MySQL WHERE id SELECT
本文参考: Mysql菜鸟教程 C语言中文网教程 李玉婷老师视频教程
1.基本查询语句
- 语法:
SELECT [字段名] FROM [表名];
- 示例
SELECT Host,User FROM user;
SELECT * FROM user;
(查询user表中所有字段)SELECT id AS "序号" FROM user;
(在查询结果中将id重命名为"序号")
2.条件查询语句
- 语法:
SELECT [字段名] FROM [表名] WHERE [筛选条件];
- 含义:通过设置筛选条件,查询出字段中符合条件的值; 筛选条件主要分为两类, 条件表达式和模糊匹配条件;通过逻辑运算符可将几条简单筛选条件复合为新的筛选条件
- 条件表达式的关键词有:
> < = != <> >= <=
- 模糊匹配的关键词有:
like
between and
in
is null
- 常用的逻辑运算符:
and
或&&
,or
或||
,not
或!
;
- 条件表达式的关键词有:
- 示例
SELECT id,name FROM user WHERE id>3;
SELECT * FROM user WHERE id>3 AND name != "lena";
SELECT name FROM user WHERE id BETWEEN 1 AND 10;
SELECT * FROM user WHERE name IN('xiaoming','John','Tom')
;SELECT * FROM user WHERE phone_num IS NOT NULL OR email IS NOT NULL;
SELECT id FROM user WHERE name LIKE _e%;
(在user表中查询name字段第二个字母为'e'的id值)
3.分组查询
- 语法:
SELECT [查询字段] FROM [表名] WHERE [筛选条件] GROUP BY[作为分组依据的字段];
- 含义:将具有某种相同性质的数据作为一个整体查询, 可查看其平均值, 总和等整体性质
- 分组可以按单个字段也可以按多个字段
- 分组条件最好放在
WHERE
筛选条件后
- 示例
SELECT COUNT(*) FROM employees WHERE department_id=90;
(无分组条件)SELECT AVG(salary),job_id FROM employees GROUP BY job_id;
SELECT COUNT(*),department_id FROM employees GROUP BY department_id;
SELECT COUNT(*),department_id FROM employees GROUP BY department_id HAVING COUNT(*)>5;
(对上述查询结果进行二次筛选)
4.排序查询
- 语法:
SELECT [查询字段] FROM [表名] ORDER BY [排序字段][排序方式];
- 含义:将查询结构按指定方式排序
- DESC (降序) ASC (升序,默认排序)
- 示例
SELECT * FROM employees ORDER BY salary;
(默认升序)SELECT * FROM employees ORDER BY salary DESC;
SELECT *,salary*12*(1+IFNULL(commission_pct,0)) FROM employees ORDER BY salary*12*(1+IFNULL(commission_pct,0)) DESC;
(按表达式排序)
5.连接查询
- 语法: `SELECT [查询字段] FROM [表名(多个)] WHERE [筛选条件];
- 含义: 连接查询一般是多表查询, 多表连接时只是简单的取笛卡尔积, 故需要设置条件对查询结果进行筛选
- 由于涉及到多个表, 故连接查询中需要指出字段所在的表,以避免名字冲突
- 示例
SELECT name,boyName FROM boys,beauty WHERE beauty.boyfriend_id= boys.id;
SELECT name,boyName FROM boys a,beauty b WHERE a.boyfriend_id= b.id;
(取别名)
6.子查询
- 含义: 出现在其他语句中的select语句,称为子查询或内查询
示例
#1.查询工资比Abel高的员工信息 SELECT * FROM employees WHERE salary>( SELECT salary FROM employees WHERE last_name = 'Abel' ); #2.查询查询员工的姓名,job_id 和工资,要求job_id=①并且salary>② SELECT last_name,job_id,salary FROM employees WHERE job_id = ( SELECT job_id FROM employees WHERE employee_id = 141 #① ) AND salary>( SELECT salary FROM employees WHERE employee_id = 143 #② ); #3.查询员工最低工资低于结果①的部门id,以及这些部门的员工最低工资 SELECT MIN(salary),department_id FROM employees GROUP BY department_id HAVING MIN(salary)>( SELECT MIN(salary) #①查询部门id为50的员工最低工资 FROM employees WHERE department_id = 50 );
7.分页查询 (不常用)
- 语法:
SELECT [查询字段] FROM [表名] limit [offset,]size;
- offset为起始值,默认为0, size为展示结果条数
- 含义:截取查询结果中的一部分展示
- 示例
SELECT * FROM employees LIMIT 0,5;
#展示查询结果中前5条SELECT * FROM employees LIMIT 5;
#同上SELECT * FROM employees LIMIT 10,15;
#展示查询结果中11-25条
8.联合查询 (不常用)
- 语法:
查询语句1 union 查询语句2 union ...
- 含义: 将几个查询语句的结果整合到一起
示例
SELECT * FROM employees WHERE email LIKE '%a%' UNION SELECT * FROM employees WHERE department_id>90; #等价于 ??? SELECT * FROM employees WHERE email LIKE '%a%' OR department_id>90;
标签:语句,salary,employees,查询,MySQL,WHERE,id,SELECT 来源: https://www.cnblogs.com/ofcwy/p/12268690.html
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