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
SELECT column1, column2, ...
FROM table_name
LIMIT number_of_rows;SELECT column1, column2, ...
FROM table_name
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
+-------+-------+----------+-------------+-----------+---------------------+
| host | idc | cpu_util | memory_util | disk_util | ts |
+-------+-------+----------+-------------+-----------+---------------------+
| host1 | idc_a | 11.8 | 10.3 | 10.3 | 2022-11-03 03:39:57 |
| host1 | idc_b | 50 | 66.7 | 40.6 | 2022-11-03 03:39:57 |
| host1 | idc_c | 50.1 | 66.8 | 40.8 | 2022-11-03 03:39:57 |
| host1 | idc_e | NULL | 66.7 | 40.6 | 2022-11-03 03:39:57 |
| host2 | idc_a | 80.1 | 70.3 | 90 | 2022-11-03 03:39:57 |
+-------+-------+----------+-------------+-----------+---------------------++-------+-------+----------+-------------+-----------+---------------------+
| host | idc | cpu_util | memory_util | disk_util | ts |
+-------+-------+----------+-------------+-----------+---------------------+
| host1 | idc_a | 11.8 | 10.3 | 10.3 | 2022-11-03 03:39:57 |
| host1 | idc_b | 50 | 66.7 | 40.6 | 2022-11-03 03:39:57 |
| host1 | idc_c | 50.1 | 66.8 | 40.8 | 2022-11-03 03:39:57 |
| host1 | idc_e | NULL | 66.7 | 40.6 | 2022-11-03 03:39:57 |
| host2 | idc_a | 80.1 | 70.3 | 90 | 2022-11-03 03:39:57 |
+-------+-------+----------+-------------+-----------+---------------------+使用 LIMIT 获取 memory_util 列中的前 3 个值：
sql
SELECT host, idc, memory_util
FROM system_metrics
ORDER BY memory_util DESC
LIMIT 3;SELECT host, idc, memory_util
FROM system_metrics
ORDER BY memory_util DESC
LIMIT 3;结果为：
sql
+-------+-------+-------------+
| host | idc | memory_util |
+-------+-------+-------------+
| host2 | idc_a | 70.3 |
| host1 | idc_c | 66.8 |
| host1 | idc_b | 66.7 |
+-------+-------+-------------++-------+-------+-------------+
| host | idc | memory_util |
+-------+-------+-------------+
| host2 | idc_a | 70.3 |
| host1 | idc_c | 66.8 |
| host1 | idc_b | 66.7 |
+-------+-------+-------------+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
SELECT host, idc, memory_util
FROM system_metrics
ORDER BY memory_util DESC
LIMIT 2 OFFSET 1;SELECT host, idc, memory_util
FROM system_metrics
ORDER BY memory_util DESC
LIMIT 2 OFFSET 1;或
sql
SELECT host, idc, memory_util
FROM system_metrics
ORDER BY memory_util DESC
LIMIT 1, 2;SELECT host, idc, memory_util
FROM system_metrics
ORDER BY memory_util DESC
LIMIT 1, 2;结果如下：
sql
+-------+-------+-------------+
| host | idc | memory_util |
+-------+-------+-------------+
| host1 | idc_c | 66.8 |
| host1 | idc_b | 66.7 |
+-------+-------+-------------++-------+-------+-------------+
| host | idc | memory_util |
+-------+-------+-------------+
| host1 | idc_c | 66.8 |
| host1 | idc_b | 66.7 |
+-------+-------+-------------+