mysql多表联查

1、在school数据库中创建两个数据表

输入user school

输入create table classes(id int primary key auto_increment,label char(30),description text) charset="utf8";

即在school数据库中创建了classes表

输入create table t_classes(id int primary key auto_increment,t_id int,c_id int,delete_flage char(4)) charset="utf8";

即在school数据库中创建了t_classes表

2、用python的pymysql模块给mysql添加数据

（1）首先给classes数据表添加数据

# coding:utf-8
import random
import pymysql

db = pymysql.connect(
    user="root",
    password="1234",
    host="localhost",
    database="school",
    charset="utf8"
)

cursor = db.cursor()
pro = ["python", "c", "java", "php", "linux"]
for index, i in enumerate(pro):
    description = "%s 是世界上第%s好的语言" % (i, index + 1)
    sql = "insert into classes(label,description) value('%s','%s')" % (i, description)
    # sql = "update classes set description='%s' where id = %s" % (description, index + 1)
    try:
        cursor.execute(sql)
    except Exception as e:
        print(e)
    else:
        print(sql)
db.commit()
cursor.close()
db.close()

即在数据表中添加了这些数据

（2）给t_classes数据表添加数据

# coding:utf-8
import random
import pymysql
from functools import partial


def exect(cursor, sql):
    try:
        cursor.execute(sql)
    except Exception as e:
        print(e)
    else:
        print(sql)


def outer(fun):
    def inner(database="school"):
        db = pymysql.connect(
            user="root",
            password="1234",
            host="localhost",
            database=database,
            charset="utf8"
        )

        cursor = db.cursor()
        sqls = fun()
        func = partial(exect, cursor)
        if isinstance(sqls, list):
            for sql in sqls:
                func(sql)
        elif isinstance(sqls, str):
            func(sql)
        db.commit()
        cursor.close()
        db.close()

    return inner


@outer
def insert_tClass():
    sqls = []
    for i in range(200):
        t_id = random.randint(1, 100)
        c_id = random.randint(1, 5)
        sql = "insert into t_classes(t_id,c_id,delete_flag) value(%s,%s,'T')" % (t_id, c_id)
        sqls.append(sql)
    return sqls


if __name__ == '__main__':
    insert_tClass()

即在数据库中添加了这些数据

3、查询

做好了上面的准备工作之后，我们可以开始查询了

（1）比如，我要查询所有python科目的老师

输入select * from teacher where project="python"

上面的个代码并没有用到多表联查，因为teacher这个表中，就有project这一项，直接就可以查，如果没有的话，我们就借助其它的方式。

输入select * from teacher where project=(select label from classes where id=1);

得到的结果是一样的，因为classes 数据表中id为1对应的字段label就是python

（2）查询teacher表中，id为1的老师的project在classes表中对应的label和description

输入select label,description from classes where label=(select project from eacher where id=1);

（3）由于操作的失误，在teacher表中出现了一个课程，是classes的字段里没有的课程，现在需要把它找出来

输入select name,project from teacher where teacher.project not in(select label from classes);

得到了结果：

然后我们把这个出错的数据删掉：

输入delete from teacher where name='lee';

（4）删除teacher表的project字段，查询ID为1的老师对应的课程

输入select c.label,c.description from classes as c,t_classes as tc where tc.t_id=1 and tc.c_id=c.id;

如果想显示名字的话可以增加显示字段

输入select t.name,c.label,description from classes as c,t_classes as tc,teacher as t where tc.t_id=1 and tc.c_id=c.id and tc.t_id=t.id;

（5）查询课程ID为1对应的老师

输入select t.name,c.label from classes as c,t_classes as tc,teacher as t where tc.c_id=1 and c.id=1 and tc.t_id=t.id;