From 62b5f5fd30b84fdcec5573fdf5ebb11b81910109 Mon Sep 17 00:00:00 2001
From: xuxin <840198532@qq.com>
Date: Sun, 11 Oct 2026 14:43:58 +0800
Subject: [PATCH] =?UTF-8?q?javaEEday07-DQL-Select-=E5=88=86=E7=BB=84?=
 =?UTF-8?q?=E6=9F=A5=E8=AF=A2?=
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---
 mysql-day02-DQL.sql | 40 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/mysql-day02-DQL.sql b/mysql-day02-DQL.sql
index 739b396..91dabc7 100644
--- a/mysql-day02-DQL.sql
+++ b/mysql-day02-DQL.sql
@@ -88,4 +88,42 @@ select * from  tb_emp where name like '%三%';
 -- 条件 : name , gender , entrydate
 select * from tb_emp where name like '%张%'
                        and gender = 1
-                       and entrydate between '2000-01-01' and '2010-01-01'
+                       and entrydate between '2000-01-01' and '2010-01-01';
+
+-- 分组查询---聚合函数
+-- 1. 统计该企业员工数量 -- count
+-- count(字段)
+select count(id) from tb_emp;
+select count(name) from tb_emp;
+select count(job) from tb_emp; -- 注意：所有的聚合函数是排除null计算的！！！！
+
+select count(*) from tb_emp;
+
+select count(0) from tb_emp;-- 统计总数推荐count(0)
+
+-- 2. 统计该企业最早入职的员工的入职日期
+select min(entrydate) from tb_emp;
+
+-- 3. 统计该企业最近入职的员工的入职日期
+select max(entrydate) from tb_emp;
+
+-- 4. 统计该企业员工的 ID 之和
+select  sum(id) from tb_emp;
+
+-- 5. 统计该企业员工 ID 的平均值
+select  avg(id) from tb_emp;
+
+-- 6. 不用avg如何计算平均值
+select sum(id)/count(0) from tb_emp;
+
+-- 分组查询
+-- 1. 根据性别分组 , 统计男性和女性员工的数量  -- count(*)
+-- 注意：分组之后能查询的字段只能是分组字段或者聚合函数才有意义
+select  gender, count(1) from tb_emp group by gender;
+
+-- 2. 先查询入职时间在 '2015-01-01' (包含) 以前的员工 , 并对结果根据职位分组 , 获取员工数量大于等于2的职位 -- count
+
+select job,count(0) from tb_emp
+where entrydate <= '2015-01-01'
+group by job
+having count(0)>= 2;