openGauss学习笔记-18 openGauss 简单数据管理-WHERE子句

news2024/11/24 15:59:16

文章目录

- openGauss学习笔记-18 openGauss 简单数据管理-WHERE子句
- - 18.1 语法格式
  - 18.2 参数说明
  - 18.3 示例

openGauss学习笔记-18 openGauss 简单数据管理-WHERE子句

当我们需要根据指定条件从表中查询数据时，就可以在SELECT语句中添加WHERE子句，从而过滤掉我们不需要数据。WHERE子句构成一个行选择表达式，用于指定条件而获取的数据，如果给定的条件满足，才返回从表中的具体数值。

18.1 语法格式

SELECT 
{ * | [column, ...] }
[ FROM from_item [, ...] ]
[ WHERE condition ];

18.2 参数说明

WHERE子句

WHERE子句构成一个行选择表达式，用来缩小SELECT查询的范围。在WHERE子句中可以使用比较运算符或逻辑运算符，例如 >, <, =, LIKE, NOT 等等。
condition

condition是返回值为布尔型的任意表达式，任何不满足该条件的行都不会被检索。

18.3 示例

使用逻辑运算符读取表customer_t1。

AND

找到c_customer_id为hello且c_customer_sk为3869的行。

openGauss=# SELECT * FROM customer_t1 WHERE  c_customer_sk = 3869 AND c_customer_id = 'hello';
 c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
          3869 | hello         | Grace        |             |   1000
          3869 | hello         | Grace        |             |   1000
          3869 | hello         |              |             |
          3869 | hello         |              |             |
(4 rows)

找到c_customer_sk大于6985或者c_customer_id为hello的行。

openGauss=# SELECT * FROM customer_t1 WHERE  c_customer_sk > 6985 OR c_customer_id = 'hello';
c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
          3869 | hello         | Grace        |             |   1000
          3869 | hello         | Grace        |             |   1000
          3869 | hello         |              |             |
          3869 | hello         |              |             |
          9976 | world         | James        |             |   5000
(5 rows)

NOT NULL

找到customer_t1中c_last_name字段不为空的行。

openGauss=# SELECT * FROM customer_t1 WHERE  c_last_name IS NOT NULL;
 c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
(0 rows)

BETWEEN

找到c_customer_sk在4000和9000之间的行。

openGauss=# SELECT * FROM customer_t1 WHERE  c_customer_sk  BETWEEN 4000 AND 9000;
 c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
          6985 | maps          | Joes         |             |   2200
          4421 | Admin         | Local        |             |   3000
(2 rows)