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SQL进阶系列之4HAVING字句的力量

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12/01
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写在前面

SQL是面向集合的语言，与面向过程和面向对象语言都不一样

寻找缺失的编号 
/* 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');


-- 如果有查询结果，说明存在缺失的编号

SELECT '存在缺失的编号' FROM SeqTbl HAVING COUNT(*) <> MAX(seq);

新的SQL标准里HAVING可以单独使用 

-- 查询缺失编号的最小值，如果表包含NULL，NOT IN可能得不到正确结果

SELECT MIN(seq+1) AS gap FROM SeqTbl WHERE (seq+1) NOT IN (SELECT seq FROM SeqTbl);

用HAVING子句进行子查询：求众数 
/* 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);


-- 求众数的SQL语句(1):使用谓词

SELECT income,COUNT(*) FROM Graduates GROUP BY income HAVING COUNT(*) >= ALL(SELECT COUNT(*) FROM Graduates GROUP BY income);


-- 求众数的SQL语句(2):使用极值函数

SELECT income,count(*) FROM Graduates GROUP BY income HAVING COUNT(*) >= (SELECT MAX(cnt) FROM (SELECT COUNT(*) as cnt FROM Graduates GROUP BY income) AS tmp);

用HAVING子句进行自连接：求中位数 
-- 求中位数的SQL语句：在HAVING子句中使用非等值自连接

SELECT AVG(income) FROM

(SELECT T1.income FROM Graduates T1,Graduates T2 GROUP BY T1.income HAVING SUM(CASE WHEN T2.income <= T1.income THEN 1 ELSE 0 END) >= COUNT(*)/2 AND SUM(CASE WHEN T2.income >= T1.income THEN 1 ELSE 0 END) >= COUNT(*)/2) AS TMP;

查询不包含NULL的集合

COUNT函数的使用方法有COUNT(*)和COUNT(<字段名>)两种，区别在于 ：

  • COUNT(*)可以用于NULL，而COUNT(<列名>)与其他聚合函数一样，要先排除掉null再进行统计
  • COUNT(*)查的是所有行的数目，而COUNT(<列名>)不一定是 
/* 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');


-- 查询"sbmt_date"列不包含NULL的列(1):使用COUNT

SELECT dpt FROM Students GROUP BY dpt HAVING COUNT(*) = COUNT(sbmt_date);


-- 查询"sbmt_date"列不包含NULL的列(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);

用关系除法进行购物篮分析 
/* 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');


-- 查到items表里商品都有的shop名称

SELECT  SI.shop FROM ShopItems AS SI,Items AS I WHERE SI.item = I.item GROUP BY SI.shop

HAVING COUNT(SI.item) = (SELECT COUNT(item) FROM Items);


-- 查找全都有且只有items表中商品的shop名称

SELECT SI.shop FROM ShopItems AS SI LEFT JOIN Items AS I ON SI.item = I.item GROUP BY SI.shop HAVING COUNT(SI.item) = (SELECT COUNT(item) FROM Items) AND

COUNT(I.item) = (SELECT COUNT(item) FROM Items);

小结
  • 表不是文件，记录也没有顺序，所以SQL不进行排序
  • SQL不是面向过程语言，没有循环、条件分支和赋值操作
  • SQL通过不断生成子集来求得目标集合
  • GROUP BY子句可以用来生成子集
  • WHERE子句用来调查集合元素的性质，而HAVING子句用来调查集合本身的性质

练习题 
-- 1-4-1 修改编号缺失的逻辑，使结果总是返回一行数据

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 col FROM SeqTbl;


-- 1-4-2 练习"特征函数"

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);


-- 1-4-3 购物篮分析问题的一般化

SELECT shop,COUNT(I.item) AS my_item_cnt,(SELECT COUNT(*) FROM Items) - COUNT(I.item) AS diff_cnt FROM ShopItems AS SI LEFT JOIN Items AS I ON SI.item = I.item GROUP BY shop;

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