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()

Output

[('Sharukh', 'Sheik', 25, 'M', 8300.0), ('Sarmista', 'Sharma', 26, 'F', 10000.0)]