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)