数据库谓词

谓词：属于函数的一种，但其返回值是真值(true/false/unknown)

判断是否存在满足某种条件的记录，存在返回TRUE、不存在返回FALSE。
比较多用到的几种谓词:

• LIKE
• BETWEEN
• IS NULL/IS NOT NULL
• IN
• EXISTS

LIKE谓词——字符串的部分一直查询(模糊查询)

--MySQL
--DDL：创建表
CREATE TABLE SampleLike
( strcol VARCHAR(6) NOT NULL,
	PRIMARY KEY (strcol));

--DML：插入数据
START TRANSACTION;
INSERT INTO SampleLike (strcol) VALUES ('abcddd');
INSERT INTO SampleLike (strcol) VALUES ('dddabc');
INSERT INTO SampleLike (strcol) VALUES ('abdddc');
INSERT INTO SampleLike (strcol) VALUES ('abcdd');
INSERT INTO SampleLike (strcol) VALUES ('ddabc');
INSERT INTO SampleLike (strcol) VALUES ('abddc');
COMMIT;

1.前方一致查询，查询对象字符串起始部分相同的记录值的查询方法。

-- 匹配以ddd开始的字符串
select * from SampleLike where strcol LIKE 'ddd%';  

2.中间一致查询

-- 匹配包含ddd的字符串
select * from SampleLike where strcol LIKE '%ddd%';  

3.后方一致查询

-- 匹配以ddd结尾的字符串
select * from SampleLike where strcol LIKE '%ddd';  

4.使用下划线_代替%,它代表的是任意一个字符

-- 匹配以abc开始的后面要有2个字符的字符串
select * from SampleLike where strcol LIKE 'abc__'; 

BETWEEN谓词——范围查询

(注：会包含100和1000两个临界值，如果不需要，需使用

-- 包含临界值100和1000
SELECT product_name, sale_price
	FROM Product
 WHERE sale_price BETWEEN 100 AND 1000;
 
-- 不包含临界值
SELECT product_name, sale_price
 FROM Product
 WHERE sale_price > 100 
 AND sale_price < 1000;

is null, is not null判断是否为空

为选取某些值为null的数据列，不能使用=，只能由is null

SELECT product_name, purchase_price
	FROM Product
 WHERE purchase_price IS NULL;

IN谓词——OR的简单用法(使用in代替or)

1.通过OR指定多个进货单价查询(阅读性较差)

SELECT product_name, purchase_price
FROM Product
 WHERE purchase_price =  320
		OR purchase_price =  500
		OR purchase_price = 5000;  

2.通过in来指定多个进货单价查询(否定形式可以使用NOT IN来实现，注：IN和NOT IN无法选出NULL数据)

SELECT product_name, purchase_price
	FROM Product
 WHERE purchase_price IN (320, 500, 5000);

3.使用子查询作为IN谓词的参数

SELECT product_name, sale_price
   FROM Product
 WHERE product_id IN (SELECT product_id 
                         FROM ShopProduct
                       WHERE shop_id = '000C');

EXIST谓词

注：实际上不使用EXIST基本上也都可以使用IN(或NOT IN)来替代

SELECT product_name, sale_price
FROM Product AS P
 WHERE EXISTS (SELECT *
				FROM ShopProduct AS SP
				WHERE SP.shop_id = '000C'
					AND SP.product_id = P.product_id);

-- 创建ShopProduct商店商品表
CREATE TABLE ShopProduct
(shop_id    CHAR(4)       NOT NULL,
 shop_name  VARCHAR(200)  NOT NULL,
 product_id CHAR(4)       NOT NULL,
 quantity   INTEGER       NOT NULL,
 PRIMARY KEY (shop_id, product_id));
 
-- MySQL
START TRANSACTION;

INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000A',	'北京',		'0001',	30);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000A',	'北京',		'0002',	50);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000A',	'北京',		'0003',	15);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000B',	'上海',	'0002',	30);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000B',	'上海',	'0003',	120);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000B',	'上海',	'0004',	20);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000B',	'上海',	'0006',	10);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000B',	'上海',	'0007',	40);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000C',	'深圳',		'0003',	20);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000C',	'深圳',		'0004',	50);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000C',	'深圳',		'0006',	90);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000C',	'深圳',		'0007',	70);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000D',	'广州',		'0001',	100);

COMMIT;