LIMIT

LIMIT 用于限制查询返回的行数。当处理大数据集时该子句特别有用，因为它通过减少需要处理的数据量来提高查询性能。

Syntax

LIMIT 的基本语法如下：

sql

  1. SELECT column1, column2, ...
  2. FROM table_name
  3. LIMIT number_of_rows;

number_of_rows 参数用于指定要返回的最大行数。如果该参数的值为负数，则不返回任何行。

示例

假如我们有一个名为 system_metrics 的表：

sql

  1. +-------+-------+----------+-------------+-----------+---------------------+
  2. | host  | idc   | cpu_util | memory_util | disk_util | ts                  |
  3. +-------+-------+----------+-------------+-----------+---------------------+
  4. | host1 | idc_a |     11.8 |        10.3 |      10.3 | 2022-11-03 03:39:57 |
  5. | host1 | idc_b |       50 |        66.7 |      40.6 | 2022-11-03 03:39:57 |
  6. | host1 | idc_c |     50.1 |        66.8 |      40.8 | 2022-11-03 03:39:57 |
  7. | host1 | idc_e |     NULL |        66.7 |      40.6 | 2022-11-03 03:39:57 |
  8. | host2 | idc_a |     80.1 |        70.3 |        90 | 2022-11-03 03:39:57 |
  9. +-------+-------+----------+-------------+-----------+---------------------+

使用 LIMIT 获取 memory_util 列中的前 3 个值：

sql

  1. SELECT host, idc, memory_util
  2. FROM system_metrics
  3. ORDER BY memory_util DESC
  4. LIMIT 3;

结果为：

sql

  1. +-------+-------+-------------+
  2. | host  | idc   | memory_util |
  3. +-------+-------+-------------+
  4. | host2 | idc_a |        70.3 |
  5. | host1 | idc_c |        66.8 |
  6. | host1 | idc_b |        66.7 |
  7. +-------+-------+-------------+

LIMIT n, m 允许在跳过前 n 行后从结果中选择 m 行，等价于LIMIT m OFFSET n 语法。

sql

  1. SELECT host, idc, memory_util
  2. FROM system_metrics
  3. ORDER BY memory_util DESC
  4. LIMIT 2 OFFSET 1;

或

sql

  1. SELECT host, idc, memory_util
  2. FROM system_metrics
  3. ORDER BY memory_util DESC
  4. LIMIT 1, 2;

结果如下：

sql

  1. +-------+-------+-------------+
  2. | host  | idc   | memory_util |
  3. +-------+-------+-------------+
  4. | host1 | idc_c |        66.8 |
  5. | host1 | idc_b |        66.7 |
  6. +-------+-------+-------------+