Python PostgreSQL Where 语句

在执行 SELECT、UPDATE 或 DELETE 操作时，你可以使用 WHERE 子句指定条件来过滤记录。该操作将在满足给定条件的记录上执行。

语法

以下是PostgreSQL中WHERE子句的语法-

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

你可以使用比较或逻辑运算符来指定搜索条件，如>、<、=、LIKE、NOT等。下面的例子可以清楚地说明这个概念。

例子

假设我们创建了一个名为CRICKETERS的表，使用以下查询-

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语句在其中插入5条记录，如 −

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

下面的SELECT语句检索了年龄大于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=#

使用python的where子句

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

例子

下面的python例子演示了使用python的WHERE命令的用法。

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()

#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()

输出

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