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;