Python Data Access 简明教程
Python PostgreSQL - Order By
通常，如果你尝试从表中检索数据，你将按照插入数据的顺序获取记录。
使用 ORDER BY 子句，在检索表记录时，你可以根据所需的列以升序或降序对结果记录进行排序。
Syntax
以下是 PostgreSQL 中 ORDER BY 子句的语法。
SELECT column-list
FROM table_name
[WHERE condition]
[ORDER BY column1, column2, .. columnN] [ASC | DESC];Example
假设我们使用以下查询创建了一个名为 CRICKETERS 的表 -
postgres=# CREATE TABLE CRICKETERS (
First_Name VARCHAR(255), Last_Name VARCHAR(255),
Age int, Place_Of_Birth VARCHAR(255), Country VARCHAR(255)
);
CREATE TABLE
postgres=#并且如果我们使用 INSERT 语句向其中插入 5 条记录，如下所示 -
postgres=# insert into CRICKETERS values('Shikhar', 'Dhawan', 33, 'Delhi', 'India');
INSERT 0 1
postgres=# insert into CRICKETERS values('Jonathan', 'Trott', 38, 'CapeTown', 'SouthAfrica');
INSERT 0 1
postgres=# insert into CRICKETERS values('Kumara', 'Sangakkara', 41, 'Matale', 'Srilanka');
INSERT 0 1
postgres=# insert into CRICKETERS values('Virat', 'Kohli', 30, 'Delhi', 'India');
INSERT 0 1
postgres=# insert into CRICKETERS values('Rohit', 'Sharma', 32, 'Nagpur', 'India');
INSERT 0 1选择语句检索 CRICKETERS 表的行按升序排列它们的年龄—
postgres=# SELECT * FROM CRICKETERS ORDER BY AGE;
first_name | last_name | age | place_of_birth | country
------------+------------+-----+----------------+-------------
Virat | Kohli | 30 | Delhi | India
Rohit | Sharma | 32 | Nagpur | India
Shikhar | Dhawan | 33 | Delhi | India
Jonathan | Trott | 38 | CapeTown | SouthAfrica
Kumara | Sangakkara | 41 | Matale | Srilanka
(5 rows)es:您可以使用多列来对表的记录进行排序。下面的 SELECT 语句根据列 age 和 FIRST_NAME 对 CRICKETERS 表的记录进行排序。
postgres=# SELECT * FROM CRICKETERS ORDER BY AGE, FIRST_NAME;
first_name | last_name | age | place_of_birth | country
------------+------------+-----+----------------+-------------
Virat | Kohli | 30 | Delhi | India
Rohit | Sharma | 32 | Nagpur | India
Shikhar | Dhawan | 33 | Delhi | India
Jonathan | Trott | 38 | CapeTown | SouthAfrica
Kumara | Sangakkara | 41 | Matale | Srilanka
(5 rows)默认情况下， ORDER BY 子句按升序对表的记录进行排序。您可以使用 DESC 按降序排列结果，如下所示：
postgres=# SELECT * FROM CRICKETERS ORDER BY AGE DESC;
first_name | last_name | age | place_of_birth | country
------------+------------+-----+----------------+-------------
Kumara | Sangakkara | 41 | Matale | Srilanka
Jonathan | Trott | 38 | CapeTown | SouthAfrica
Shikhar | Dhawan | 33 | Delhi | India
Rohit | Sharma | 32 | Nagpur | India
Virat | Kohli | 30 | Delhi | India
(5 rows)ORDER BY clause using python
要以特定顺序检索表的全表，请对光标对象调用 execute() 方法，并将 SELECT 语句连同 ORDER BY 子句作为参数传递给它。
Example
在下面的示例中，我们使用 ORDER BY 子句创建了一个名为 Employee 的表，填充它，并按年龄（升序）检索其记录。
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()
#Doping EMPLOYEE table if already exists.
cursor.execute("DROP TABLE IF EXISTS EMPLOYEE")
#Creating a table
sql = '''CREATE TABLE EMPLOYEE(
FIRST_NAME CHAR(20) NOT NULL,
LAST_NAME CHAR(20),
AGE INT, SEX CHAR(1),
INCOME INT,
CONTACT INT
)'''
cursor.execute(sql)
#Populating the table
insert_stmt = "INSERT INTO EMPLOYEE (
FIRST_NAME, LAST_NAME, AGE, SEX, INCOME, CONTACT) VALUES
(%s, %s, %s, %s, %s, %s)"
data = [('Krishna', 'Sharma', 26, 'M', 2000, 101),
('Raj', 'Kandukuri', 20, 'M', 7000, 102),
('Ramya', 'Ramapriya', 29, 'F', 5000, 103),
('Mac', 'Mohan', 26, 'M', 2000, 104)]
cursor.executemany(insert_stmt, data)
conn.commit()
#Retrieving specific records using the ORDER BY clause
cursor.execute("SELECT * from EMPLOYEE ORDER BY AGE")
print(cursor.fetchall())
#Commit your changes in the database
conn.commit()
#Closing the connection
conn.close()