SQL基础教程（第2版）第6章函数、谓词、CASE表达式：6-2 谓词

2023-02-25 12:48:19

● 谓词就是返回值为真值的函数。
● 可以将子查询作为IN和EXISTS的参数。

本节将会和大家一起学习 SQL 的抽出条件中不可或缺的工具——谓词（predicate）。例如， =、 <、 >、 <> 等比较运算符，其正式的名称就是比较谓词。

谓词的返回值全都是真值（TRUE/FALSE/UNKNOWN）。这也是谓词和函数的最大区别。

● LIKE
● BETWEEN
● IS NULL、 IS NOT NULL
● IN
● EXISTS

LIKE谓词——字符串的部分一致查询
部分一致大体可以分为前方一致、中间一致和后方一致三种类型。

--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;

List6_21_MySQL

--Oracle, DB2
--DDL：创建表
CREATE TABLE SampleLike
( strcol VARCHAR(6) NOT NULL,
  PRIMARY KEY (strcol));

--DML：插入数据

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;

List6_21_Oracle_DB2

BETWEEN谓词——范围查询

IS NULL、 IS NOT NULL——判断是否为NULL

IN谓词——OR的简便用法

但需要注意的是，在使用IN 和 NOT IN 时是无法选取出NULL 数据的。
NULL 终究还是需要使用IS NULL 和IS NOT NULL 来进行判断。

使用子查询作为IN谓词的参数
■ IN和子查询
IN 谓词（NOT IN 谓词）具有其他谓词所没有的用法，那就是可以
使用子查询作为其参数。我们已经在 5-2 节中学习过了，子查询就是 SQL
内部生成的表，因此也可以说“能够将表作为 IN 的参数”。同理，我们
还可以说“能够将视图作为 IN 的参数”。

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',    '东京',        '0001',    30);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000A',    '东京',        '0002',    50);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000A',    '东京',        '0003',    15);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000B',    '名古屋',    '0002',    30);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000B',    '名古屋',    '0003',    120);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000B',    '名古屋',    '0004',    20);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000B',    '名古屋',    '0006',    10);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000B',    '名古屋',    '0007',    40);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000C',    '大阪',        '0003',    20);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000C',    '大阪',        '0004',    50);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000C',    '大阪',        '0006',    90);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000C',    '大阪',        '0007',    70);
INSERT INTO ShopProduct (shop_id, shop_name, product_id, quantity) VALUES ('000D',    '福冈',        '0001',    100);

COMMIT;

代码清单6-35　向ShopProduct表中插入数据