javaEEday07-DQL-Select-基本查询

mysql-day02.sql

@@ -17,7 +17,7 @@ create table tb_emp
         unique (username)
 )
     comment '员工表';
-
+-- ========================================基本查询==========================================
 -- DML:数据操作语言
 -- DML : 插入数据 - insert
 -- 1. 为 tb_emp 表的 username, name, gender 字段插入值
@@ -58,3 +58,27 @@ delete from tb_emp where id = 1;
 
 -- 2. 删除 tb_emp 表中的所有员工
 delete from tb_emp;
+
+-- ======================================DQL===================================
+-- 基本查询
+-- 1. 查询指定字段 name,entrydate 并返回
+select name,entrydate from tb_emp;
+
+-- 2. 查询返回所有字段
+-- 方式一:  推荐 , 效率高 . 更直观
+select  id, username, password, name, gender,
+        image, job, entrydate, dept_id,
+        create_time, update_time from tb_emp;
+
+-- 方式二:不推荐（不直观，性能低）
+select * from tb_emp;
+
+-- 3. 查询所有员工的 name,entrydate, 并起别名(姓名、入职日期)  --- as 关键字可以省略
+select name as '姓名',entrydate as '入职日期' from tb_emp;
+
+select name  '姓名',entrydate  '入职日期' from tb_emp;
+
+-- 4. 查询员工有哪几种职位(不要重复) -- distinct
+select distinct job from tb_emp;
+
+