SQL进阶之HAVING用法

• 一、HAVING子句简介
• 二、HAVING实操
• 1.CreateTable
• 2.Sample

一、HAVING子句简介

HAVING是在sql的运行完后做的一个判断常跟聚合写法后面

二、HAVING实操

注:版本使用:mysql5.1.73
以下代码均可直接cv在mysql练习

1.CreateTable

/* 寻找缺失的编号 */
CREATE TABLE SeqTbl
(seq  INTEGER PRIMARY KEY,
name VARCHAR(16) NOT NULL);

INSERT INTO SeqTbl VALUES(1,	'迪克');
INSERT INTO SeqTbl VALUES(2,	'安');
INSERT INTO SeqTbl VALUES(3,	'莱露');
INSERT INTO SeqTbl VALUES(5,	'卡');
INSERT INTO SeqTbl VALUES(6,	'玛丽');
INSERT INTO SeqTbl VALUES(8,	'本');

/* 用HAVING子句进行子查询：求众数(求中位数时也用本代码) */
CREATE TABLE Graduates
(name   VARCHAR(16) PRIMARY KEY,
income INTEGER NOT NULL);

INSERT INTO Graduates VALUES('桑普森', 400000);
INSERT INTO Graduates VALUES('迈克',     30000);
INSERT INTO Graduates VALUES('怀特',   20000);
INSERT INTO Graduates VALUES('阿诺德', 20000);
INSERT INTO Graduates VALUES('史密斯',     20000);
INSERT INTO Graduates VALUES('劳伦斯',   15000);
INSERT INTO Graduates VALUES('哈德逊',   15000);
INSERT INTO Graduates VALUES('肯特',     10000);
INSERT INTO Graduates VALUES('贝克',   10000);
INSERT INTO Graduates VALUES('斯科特',   10000);

/* 用关系除法运算进行购物篮分析 */
CREATE TABLE Items
(item VARCHAR(16) PRIMARY KEY);

CREATE TABLE ShopItems
(shop VARCHAR(16),
item VARCHAR(16),
PRIMARY KEY(shop, item));

INSERT INTO Items VALUES('啤酒');
INSERT INTO Items VALUES('纸尿裤');
INSERT INTO Items VALUES('自行车');

INSERT INTO ShopItems VALUES('仙台',  '啤酒');
INSERT INTO ShopItems VALUES('仙台',  '纸尿裤');
INSERT INTO ShopItems VALUES('仙台',  '自行车');
INSERT INTO ShopItems VALUES('仙台',  '窗帘');
INSERT INTO ShopItems VALUES('东京',  '啤酒');
INSERT INTO ShopItems VALUES('东京',  '纸尿裤');
INSERT INTO ShopItems VALUES('东京',  '自行车');
INSERT INTO ShopItems VALUES('大阪',  '电视');
INSERT INTO ShopItems VALUES('大阪',  '纸尿裤');
INSERT INTO ShopItems VALUES('大阪',  '自行车');

/* 查询不包含NULL的集合 */
CREATE TABLE Students
(student_id   INTEGER PRIMARY KEY,
dpt          VARCHAR(16) NOT NULL,
sbmt_date    DATE);

INSERT INTO Students VALUES(100,  '理学院',   '2005-10-10');
INSERT INTO Students VALUES(101,  '理学院',   '2005-09-22');
INSERT INTO Students VALUES(102,  '文学院',   NULL);
INSERT INTO Students VALUES(103,  '文学院',   '2005-09-10');
INSERT INTO Students VALUES(200,  '文学院',   '2005-09-22');
INSERT INTO Students VALUES(201,  '工学院',   NULL);
INSERT INTO Students VALUES(202,  '经济学院', '2005-09-25');

2.Sample

/* 需求：修改编号缺失的检查逻辑，使结果总是返回一行数据 */
/* 写法一：union+having*/
SELECT ' 存在缺失的编号' AS gap
FROM SeqTbl
HAVING COUNT(*) <> MAX(seq)
UNION ALL
SELECT ' 不存在缺少的编号' AS gap
FROM SeqTbl
HAVING COUNT(*) = MAX(seq);

/* 写法二:case when*/
SELECT CASE WHEN COUNT(*) <> MAX(seq)
THEN '存在缺失的编号'
ELSE '不存在缺失的编号' END AS gap
FROM SeqTbl;

/*查询缺失编号的最小值*/
SELECT MIN(seq+1) AS gap
FROM SeqTbl
WHERE seq is not null and (seq+1) NOT IN (SELECT seq FROM SeqTbl)
注:若是seq存在空值就会出现没有结果的情况,使用 not in 一定要注意null

/* 练习“特征函数”
查找所有学生都在9月份提交完成的学院（1）：使用BETWEEN谓词 */
SELECT dpt
FROM Students
GROUP BY dpt
HAVING COUNT(*) = SUM(CASE WHEN sbmt_date BETWEEN '2005-09-01' AND '2005-09-30'
THEN 1 ELSE 0 END);

/* 求众数的SQL语句（1）：使用谓词all */
SELECT income, COUNT(*) AS cnt
FROM Graduates
GROUP BY income
HAVING COUNT(*) >= ALL ( SELECT COUNT(*)
FROM Graduates
GROUP BY income);
注:使用谓词ALL,在值为null或空集时也会出现问题可以使用极值代替

/* 求众数的SQL语句(2)：使用极值函数 */
SELECT income, COUNT(*) AS cnt
FROM Graduates
GROUP BY income
HAVING COUNT(*) >=  ( SELECT MAX(cnt)
FROM ( SELECT COUNT(*) AS cnt
FROM Graduates
GROUP BY income) TMP) ;

/* 求中位数的SQL语句：在HAVING子句中使用非等值自连接 */
SELECT AVG(DISTINCT income)
FROM (SELECT T1.income
FROM Graduates T1, Graduates T2
GROUP BY T1.income
/* S1的条件 */
HAVING SUM(CASE WHEN T2.income >= T1.income THEN 1 ELSE 0 END)
>= COUNT(*) / 2
/* S2的条件 */
AND SUM(CASE WHEN T2.income <= T1.income THEN 1 ELSE 0 END)
>= COUNT(*) / 2 ) TMP;

/*需求:查询“提交日期”列内不包含NULL的学院*/
/* (1)：使用COUNT函数 */
SELECT dpt
FROM Students
GROUP BY dpt
HAVING COUNT(*) = COUNT(sbmt_date);

/* (2)：使用CASE表达式 */
SELECT dpt
FROM Students
GROUP BY dpt
HAVING COUNT(*) = SUM(CASE WHEN sbmt_date IS NOT NULL
THEN 1
ELSE 0 END);

/*需求: 查询啤酒、纸尿裤和自行车同时在库的店铺 */
/* 错误的SQL语句 */
SELECT DISTINCT shop
FROM ShopItems
WHERE item IN (SELECT item FROM Items);
注:错误原因in的话只要满足其一都包含,这时候可以用HAVING来来解决问题
/* 正确的SQL语句 */
SELECT SI.shop
FROM ShopItems SI, Items I
WHERE SI.item = I.item
GROUP BY SI.shop
HAVING COUNT(SI.item) = (SELECT COUNT(item) FROM Items);
注:如果把HAVING COUNT(SI.item) = (SELECT COUNT(item) FROM Items)改成HAVING COUNT(SI.item)=COUNT(I.item),结果就会有问题,原因是受了连接操作的影响
例:/* COUNT(I.item)的值已经不一定是3了 */
SELECT SI.shop, COUNT(SI.item), COUNT(I.item)
FROM ShopItems SI, Items I
WHERE SI.item = I.item
GROUP BY SI.shop;

/* 精确关系除法运算：使用外连接和COUNT函数 */
SELECT SI.shop
FROM ShopItems AS SI LEFT OUTER JOIN Items AS I
ON SI.item=I.item
GROUP BY SI.shop
HAVING COUNT(SI.item) = (SELECT COUNT(item) FROM Items)   /* 条件1 */
AND COUNT(I.item)  = (SELECT COUNT(item) FROM Items);  /* 条件2 */

/* 购物篮分析问题的一般变化
select 嵌套+子查询*/
SELECT SI.shop,
COUNT(SI.item) AS my_item_cnt,
(SELECT COUNT(item) FROM Items) - COUNT(SI.item) AS diff_cnt
FROM ShopItems SI, Items I
WHERE SI.item = I.item
GROUP BY SI.shop;

参考资料:《SQL进阶教程》| MICK