08-基础SQL-DQL(数据查询语言)-条件查询（WHERE）-526互联

DQL-介绍（常用）

DQL英文全称是Data Query Language（数据查询语言），数据查询语言用来查询数据库中表的记录

查询关键字：SELECT

DQL-语法

DQL-条件查询

语法：

SELECT 字段列表 FROM 表名 WHERE 条件列表;

条件：

LIKE查询（模糊查询）

% 包含零个或多个字符的任意字符串

_（下划线）任何单个字符

案例：创建一个 emp 员工表，添加一些员工数据

CREATE TABLE emp(
    id int comment "编号",
    workno varchar(10) comment "工号",
    name varchar(10) comment "姓名",
    gender char(1) comment "性别",
    age tinyint unsigned comment "年龄",
    idcard char(18) comment "身份证号",
    workaddress varchar(50) comment "工作地址",
    entrydate date comment "入职时间"
) comment "员工表";

INSERT INTO emp (id,workno,name,gender,age,idcard,workaddress,entrydate)
VALUES    
(1,"1","马铃薯","男","23","123456789000000000","江西","2023-12-31"),
(2,"2","任先生","男","26","123456789000000001","河北","2023-12-31"),
(3,"3","王校凯","男","23","123456789000000002","河北","2023-12-31"),
(4,"4","张晓峰","男","24",null,"山西","2023-12-31"),
(5,"5","张三","男","18","123456789000000003","北京","2023-12-31"),
(6,"6","李四","男","18",null,"北京","2023-12-31")

1）查询年龄等于23的员工信息

SELECT * FROM emp WHERE age = 23;

2）查询年龄小于25的员工信息

SELECT * FROM emp WHERE age < 25;

3）查询没有身份证号的员工信息

SELECT * FROM emp WHERE idcard is null;

4）查询有身份证号的员工信息

SELECT * FROM emp WHERE idcard is not null;

5）查询年龄在24岁（包含）到26岁（包含）之间的员工信息]

SELECT * FROM emp WHERE age >= 24 AND age <= 26;

# 或

SELECT * FROM emp WHERE age BETWEEN 24 AND 26;

6）查询性别为 "男" , 且年龄小于24岁的员工信息

SELECT * FROM emp WHERE gender = "男" AND age <= 24;

7）查询年龄等于 18 或 24 或 26 的员工信息

SELECT * FROM emp WHERE age = 18 or age = 24 or age = 26;

# 或

SELECT * FROM emp WHERE age in (18,24,26);

8）查询姓名为两个字的员工信息

SELECT * FROM emp WHERE name LIKE "__";