database join

本文基于 sqlite 进行测试，并做如下准备工作

import sqlite3

### 连接
conn = sqlite3.connect('test.db')
print("Opened database successfully")

### 获取游标并建表
cur = conn.cursor()
cur.execute('''CREATE TABLE COMPANY
       (ID INT PRIMARY KEY     NOT NULL,
       NAME           TEXT    NOT NULL,
       AGE            INT     NOT NULL,
       ADDRESS        CHAR(50),
       SALARY         REAL);''')
print("Table created successfully")

### 写入数据
cur.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) 
      VALUES (1, 'Paul', 32, 'California', 20000.00 )")
cur.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) 
      VALUES (2, 'Allen', 25, 'Texas', 15000.00 )")
cur.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) 
      VALUES (3, 'Teddy', 23, 'Norway', 20000.00 )")
cur.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) 
      VALUES (4, 'Mark', 25, 'Rich-Mond ', 65000.00 )")

conn.commit()
conn.close()

内连接 inner join

不以谁为准，相当于取交集

sql = '''select * from company m inner join (select * from company where age > 25) n on m.age = n.age'''
cur.execute(sql)
print(cur.fetchall())
# (1, 'Paul', 32, 'California', 20000.0, 1, 'Paul', 32, 'California', 20000.0)

外连接 outer join

外连接分为 left join、right join、full join 3 种

### left join
## 以左为准，左边有 x 行，输出 x
# as 可有可无
sql = '''select * from company m left join (select * from company where age > 25) as n on m.age = n.age;'''
cur.execute(sql)
print(cur.fetchall())
# (1, 'Paul', 32, 'California', 20000.0, 1, 'Paul', 32, 'California', 20000.0)
# (2, 'Allen', 25, 'Texas', 15000.0, None, None, None, None, None)
# (3, 'Teddy', 23, 'Norway', 20000.0, None, None, None, None, None)
# (4, 'Mark', 25, 'Rich-Mond ', 65000.0, None, None, None, None, None)

### right join
## 以右为准，sqlite 不支持
sql = '''select * from company m right join (select * from company where age > 25) as n on m.age = n.age;'''

### full join
## 取并集，sqlite 不支持

交叉连接 cross join

相当于作笛卡尔内积，如左表 x 行，右表 y 行，输出 x*y 行，然后基于关键字如 on 进行筛选

sql = '''select * from company m cross join (select * from company where age > 23) n on m.age = n.age'''
cur.execute(sql)
print(cur.fetchall())
# (1, 'Paul', 32, 'California', 20000.0, 1, 'Paul', 32, 'California', 20000.0)
# (2, 'Allen', 25, 'Texas', 15000.0, 2, 'Allen', 25, 'Texas', 15000.0)
# (2, 'Allen', 25, 'Texas', 15000.0, 4, 'Mark', 25, 'Rich-Mond ', 65000.0)
# (4, 'Mark', 25, 'Rich-Mond ', 65000.0, 2, 'Allen', 25, 'Texas', 15000.0)
# (4, 'Mark', 25, 'Rich-Mond ', 65000.0, 4, 'Mark', 25, 'Rich-Mond ', 65000.0)

关键字表达式

包括 on using natural 3 种，这三个关键字不可同时使用

### using
## 简化，去重
sql = '''select * from company m cross join (select * from company where age > 23) n using (name, age);'''
cur.execute(sql)
print(cur.fetchall())
# (1, 'Paul', 32, 'California', 20000.0, 1, 'California', 20000.0)
# (2, 'Allen', 25, 'Texas', 15000.0, 2, 'Texas', 15000.0)
# (4, 'Mark', 25, 'Rich-Mond ', 65000.0, 4, 'Rich-Mond ', 65000.0)

参考资料：

https://www.runoob.com/sqlite/sqlite-joins.html　　SQLite Join

https://www.cnblogs.com/assasion/p/7768931.html　　inner join 与 left join 之间的区别