Python Data Access 简明教程

Python PostgreSQL - Where Clause

在执行SELECT、UPDATE或DELETE操作时,可以使用WHERE子句指定条件以筛选记录。该操作将对满足给定条件的记录执行。

Syntax

以下是PostgreSQL中WHERE子句的语法 −

SELECT column1, column2, columnN
FROM table_name
WHERE [search_condition]

您可以使用比较或逻辑运算符(如>、<、=、LIKE、NOT等)指定search_condition。以下示例将阐明此概念。

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

以下SELECT语句检索年龄大于35岁的记录 −

postgres=# SELECT * FROM CRICKETERS WHERE AGE > 35;
first_name  | last_name  | age | place_of_birth | country
------------+------------+-----+----------------+-------------
Jonathan    | Trott      | 38  | CapeTown       | SouthAfrica
Kumara      | Sangakkara | 41  | Matale         | Srilanka
(2 rows)
postgres=#

Where clause using python

要使用python程序从表中获取特定记录,请使用WHERE子句执行SELECT语句,通过将其作为参数传递给 execute() 方法。

Example

以下python示例演示了使用python的WHERE命令。

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")
sql = '''CREATE TABLE EMPLOYEE(
   FIRST_NAME CHAR(20) NOT NULL,
   LAST_NAME CHAR(20),
   AGE INT,
   SEX CHAR(1),
   INCOME FLOAT
)'''
cursor.execute(sql)

#Populating the table
insert_stmt = "INSERT INTO EMPLOYEE (FIRST_NAME, LAST_NAME, AGE, SEX, INCOME)
   VALUES (%s, %s, %s, %s, %s)"
data = [('Krishna', 'Sharma', 19, 'M', 2000),
   ('Raj', 'Kandukuri', 20, 'M', 7000),
   ('Ramya', 'Ramapriya', 25, 'M', 5000),
   ('Mac', 'Mohan', 26, 'M', 2000)]
cursor.executemany(insert_stmt, data)

#Retrieving specific records using the where clause
cursor.execute("SELECT * from EMPLOYEE WHERE AGE <23")
print(cursor.fetchall())

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

#Closing the connection
conn.close()

Output

[('Krishna', 'Sharma', 19, 'M', 2000.0), ('Raj', 'Kandukuri', 20, 'M', 7000.0)]