Python PostgreSQL - 加入

---

将数据分为两个表后，您可以使用联接从这两个表中获取组合记录。

例子

假设我们创建了一个名为 CRICKETERS 的表，并向其中插入了 5 条记录，如下所示 -

postgres=# CREATE TABLE CRICKETERS (
   First_Name VARCHAR(255), Last_Name VARCHAR(255), Age int, 
   Place_Of_Birth VARCHAR(255), Country VARCHAR(255)
);
postgres=# insert into CRICKETERS values (
   'Shikhar', 'Dhawan', 33, 'Delhi', 'India'
);
postgres=# insert into CRICKETERS values (
   'Jonathan', 'Trott', 38, 'CapeTown', 'SouthAfrica'
);
postgres=# insert into CRICKETERS values (
   'Kumara', 'Sangakkara', 41, 'Matale', 'Srilanka'
);
postgres=# insert into CRICKETERS values (
   'Virat', 'Kohli', 30, 'Delhi', 'India'
);
postgres=# insert into CRICKETERS values (
   'Rohit', 'Sharma', 32, 'Nagpur', 'India'
);

并且，如果我们创建了另一个名为 OdiStats 的表，并向其中插入了 5 条记录：

postgres=# CREATE TABLE ODIStats (
   First_Name VARCHAR(255), Matches INT, Runs INT, AVG FLOAT, 
   Centuries INT, HalfCenturies INT
);
postgres=# insert into OdiStats values ('Shikhar', 133, 5518, 44.5, 17, 27);
postgres=# insert into OdiStats values ('Jonathan', 68, 2819, 51.25, 4, 22);
postgres=# insert into OdiStats values ('Kumara', 404, 14234, 41.99, 25, 93);
postgres=# insert into OdiStats values ('Virat', 239, 11520, 60.31, 43, 54);
postgres=# insert into OdiStats values ('Rohit', 218, 8686, 48.53, 24, 42);

以下语句检索组合这两个表中的值的数据 -

postgres=# SELECT
Cricketers.First_Name, Cricketers.Last_Name, Cricketers.Country,
OdiStats.matches, OdiStats.runs, OdiStats.centuries, OdiStats.halfcenturies
from Cricketers INNER JOIN OdiStats ON Cricketers.First_Name = OdiStats.First_Name;
first_name  | last_name  | country     | matches | runs  | centuries | halfcenturies
------------+------------+-------------+---------+-------+-----------+---------------
Shikhar     | Dhawan     | India       | 133     | 5518  | 17        | 27
Jonathan    | Trott      | SouthAfrica | 68      | 2819  | 4         | 22
Kumara      | Sangakkara | Srilanka    | 404     | 14234 | 25        | 93
Virat       | Kohli      | India       | 239     | 11520 | 43        | 54
Rohit       | Sharma     | India       | 218     | 8686  | 24        | 42
(5 rows)
postgres=#

使用 python 加入

将数据分为两个表后，您可以使用联接从这两个表中获取组合记录。

例子

以下 python 程序演示了 JOIN 子句的用法 -

import psycopg2

#establishing the connection
conn = psycopg2.connect(
   database="mydb", user='postgres', password='password', host='127.0.0.1', port= '5432'
)
#Setting auto commit false
conn.autocommit = True

#Creating a cursor object using the cursor() method
cursor = conn.cursor()

#Retrieving single row
sql = '''SELECT * from EMP INNER JOIN CONTACT ON EMP.CONTACT = CONTACT.ID'''

#Executing the query
cursor.execute(sql)

#Fetching 1st row from the table
result = cursor.fetchall();
print(result)

#Commit your changes in the database
conn.commit()

#Closing the connection
conn.close()

输出

[('Ramya', 'Rama priya', 27, 'F', 9000.0, 101, 101, 'Krishna@mymail.com', 'Hyderabad'), 
   ('Vinay', 'Battacharya', 20, 'M', 6000.0, 102, 102, 'Raja@mymail.com', 'Vishakhapatnam'), 
   ('Sharukh', 'Sheik', 25, 'M', 8300.0, 103, 103, 'Krishna@mymail.com ', 'Pune'), 
   ('Sarmista', 'Sharma', 26, 'F', 10000.0, 104, 104, 'Raja@mymail.com', 'Mumbai')]