📅  最后修改于: 2020-11-07 07:56:51             🧑  作者: Mango
将数据分为两个表后,可以使用Joins从这两个表中获取组合记录。
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称为OdiStats的表并将5条记录插入-
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);
以下语句检索结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=#
将数据分为两个表后,可以使用Joins从这两个表中获取组合记录。
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()
[
('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')
]