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。

语法格式

  1. SELECT 
  2. { * | [column, ...] }
  3. [ FROM from_item [, ...] ]
  4. [ WHERE condition ];

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。

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。

    1. openGauss=# SELECT * FROM customer_t1 WHERE  c_customer_sk = 3869 AND c_customer_id = 'hello';
    2.  c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
    3. ---------------+---------------+--------------+-------------+--------
    4.           3869 | hello         | Grace        |             |   1000
    5.           3869 | hello         | Grace        |             |   1000
    6.           3869 | hello         |              |             |
    7.           3869 | hello         |              |             |
    8. (4 rows)

  • OR

    找到c_customer_sk大于6985或者c_customer_id为hello的行。

    1. openGauss=# SELECT * FROM customer_t1 WHERE  c_customer_sk > 6985 OR c_customer_id = 'hello';
    2. c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
    3. ---------------+---------------+--------------+-------------+--------
    4.           3869 | hello         | Grace        |             |   1000
    5.           3869 | hello         | Grace        |             |   1000
    6.           3869 | hello         |              |             |
    7.           3869 | hello         |              |             |
    8.           9976 | world         | James        |             |   5000
    9. (5 rows)

  • NOT NULL

    找到customer_t1中c_last_name字段不为空的行。

    1. openGauss=# SELECT * FROM customer_t1 WHERE  c_last_name IS NOT NULL;
    2.  c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
    3. ---------------+---------------+--------------+-------------+--------
    4. (0 rows)

  • BETWEEN

    找到c_customer_sk在4000和9000之间的行。

    1. openGauss=# SELECT * FROM customer_t1 WHERE  c_customer_sk  BETWEEN 4000 AND 9000;
    2.  c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
    3. ---------------+---------------+--------------+-------------+--------
    4.           6985 | maps          | Joes         |             |   2200
    5.           4421 | Admin         | Local        |             |   3000
    6. (2 rows)