07-基础SQL-DQL(数据查询语言)-基础查询

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-语法（编写顺序和执行顺序）

DQL-基本查询

查询多个字段

SELECT 字段1, 字段2, 字段3... FROM 表名;

SELECT * FROM 表名;

设置别名

SELECT 字段1 [AS 别名1], 字段2 [AS 别名2] ... FROM 表名;

去除重复记录

SELECT DISTINCT 字段列表 FROM 表名;

 

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","123456789000000003","山西","2023-12-31")

 1）查询指定字段 name, workno, age 返回

SELECT name, workno, age FROM emp;

2）查询所有员工的工作地址，起别名

SELECT workaddress AS"工作地址" FROM emp;

# 或者

SELECT workaddress "工作地址" FROM emp;

3）查询公司员工的工作地址（不要重复）

SELECT DISTINCT workaddress AS "工作地址" FROM emp;