Python PostgreSQL - 排序 (Order By)

通常情况下，如果您尝试从表中检索数据，您将按照插入数据的顺序获得记录。

使用ORDER BY子句，在检索表记录时，您可以根据所需的列对结果记录进行升序或降序排序。

语法

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];

示例

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=#

如果我们使用INSERT语句向其中插入了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

以下SELECT语句按年龄升序检索CRICKETERS表的行：

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:

您可以使用多个列来对表的记录进行排序。以下SELECT语句根据age和FIRST_NAME列对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子句按升序对表中的记录进行排序。您可以使用DESC来按降序排列结果，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)

使用Python的ORDER BY子句

要以特定顺序检索表的内容，请在游标对象上调用execute()方法，并将包含ORDER BY子句的SELECT语句作为参数传递给它。

示例

在下面的示例中，我们创建一个名为Employee的表，填充它，并使用ORDER BY子句按年龄（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()

输出

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

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