mysql数据库的多表查询，子查询，联合查询

/*多表查询*/
/*交叉连接*(表1条数*表2条数)*/
SELECT * FROM emp JOIN dept;
/*内连接(在交叉连接基础上加条件）*/
SELECT * FROM emp JOIN dept ON deptid=id;
SELECT * FROM emp JOIN dept ON emp.deptid=dept.id;
/*左外连接*(以左表为主表)*/；
SELECT * FROM product_type LEFT JOIN product ON product.`protype_id`=product_type.`protype_id`;
/*右外连接*(以右表为主表)*/；
SELECT * FROM product_type RIGHT JOIN product ON product.`protype_id`=product_type.`protype_id`;

/*子查询：把一个查询结果当作另一个查询的条件*/
/*找出索尼手机所属类别*/
SELECT protype_id FROM product WHERE pro_name LIKE '%索尼%4G手机%';
SELECT protype_name FROM product_type WHERE protype_id=(
SELECT protype_id FROM product WHERE pro_name LIKE '%索尼%4G手机%'
);
/*找出所有属于手机数码的产品*/
SELECT * FROM product WHERE protype_id=(
SELECT protype_id FROM product_type WHERE protype_name='手机数码'
);
/*找出所有属于手机数码或者电脑办公的产品*/
SELECT protype_id FROM product_type WHERE protype_name='手机数码' OR protype_name='电脑办公';
SELECT * FROM product WHERE protype_id IN(
SELECT protype_id FROM product_type WHERE protype_name='手机数码' OR protype_name='电脑办公'
);

/*联合查询*/
CREATE TABLE app1(
    aid INT PRIMARY KEY AUTO_INCREMENT,
    aname VARCHAR(10),
    adesc VARCHAR(50)
);
CREATE TABLE bpp1(
    bid INT PRIMARY KEY AUTO_INCREMENT,
    bname VARCHAR(10),
    bdesc VARCHAR(50)
);
INSERT INTO app1(aname,adesc) VALUES('熊大','保护森林');
INSERT INTO app1(aname,adesc) VALUES('光头强','砍树');
INSERT INTO bpp1(bname,bdesc) VALUES('小红帽','傻吊');
INSERT INTO bpp1(bname,bdesc) VALUES('大灰狼','牛逼');
SELECT aid,adesc FROM app1
UNION
SELECT bname,bdesc FROM bpp1;