Python PostgreSQL - 限制

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执行 PostgreSQL SELECT 语句时，您可以使用 LIMIT 子句限制其结果中的记录数。

句法

以下是 PostgreSQL 中 LMIT 子句的语法 -

SELECT column1, column2, columnN
FROM table_name
LIMIT [no of rows]

例子

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 语句插入 5 条记录：

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

以下语句使用 LIMIT 子句检索 Cricketers 表的前 3 条记录 -

postgres=# SELECT * FROM CRICKETERS LIMIT 3;
 first_name | last_name  | age | place_of_birth | country
------------+------------+-----+----------------+-------------
 Shikhar    | Dhawan     | 33  | Delhi          | India
 Jonathan   | Trott      | 38  | CapeTown       | SouthAfrica
 Kumara     | Sangakkara | 41  | Matale         | Srilanka

   (3 rows)

如果您想获取从特定记录（偏移量）开始的记录，可以使用 OFFSET 子句和 LIMIT 来实现。

postgres=# SELECT * FROM CRICKETERS LIMIT 3 OFFSET 2;
 first_name | last_name  | age | place_of_birth | country
------------+------------+-----+----------------+----------
 Kumara     | Sangakkara | 41  | Matale         | Srilanka
 Virat      | Kohli      | 30  | Delhi          | India
 Rohit      | Sharma     | 32  | Nagpur         | India

   (3 rows)
postgres=#

使用 Python 的限制子句

以下 python 示例检索名为 EMPLOYEE 的表的内容，将结果中的记录数限制为 2 -

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()

#Retrieving single row
sql = '''SELECT * from EMPLOYEE LIMIT 2 OFFSET 2'''

#Executing the query
cursor.execute(sql)

#Fetching the data
result = cursor.fetchall();
print(result)

#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)]