1.在MySQL中，我們要查看前3條記錄的話，是使用limit
mysql> SELECT * FROM kettle.r_step_attribute order by ID_STEP_ATTRIBUTE limit 3; +-------------------+-------------------+---------+------+---------------------+-----------+------------------------------------------------------+ | ID_STEP_ATTRIBUTE | ID_TRANSFORMATION | ID_STEP | NR | CODE | VALUE_NUM | VALUE_STR | +-------------------+-------------------+---------+------+---------------------+-----------+------------------------------------------------------+ | 1 | 1 | 1 | 0 | PARTITIONING_SCHEMA | 0 | NULL | | 2 | 1 | 1 | 0 | PARTITIONING_METHOD | 0 | none | | 3 | 1 | 1 | 0 | sql | 0 | SELECT <values> FROM <table name> WHERE <conditions> | +-------------------+-------------------+---------+------+---------------------+-----------+------------------------------------------------------+ 3 rows in set (0.00 sec) mysql>
2.簡單的分頁也可以使用力limit來完成
mysql> select * from kettle.r_step_attribute order by ID_STEP_ATTRIBUTE limit 3 offset 3; +-------------------+-------------------+---------+------+------------------+-----------+-----------+ | ID_STEP_ATTRIBUTE | ID_TRANSFORMATION | ID_STEP | NR | CODE | VALUE_NUM | VALUE_STR | +-------------------+-------------------+---------+------+------------------+-----------+-----------+ | 4 | 1 | 1 | 0 | limit | 0 | 0 | | 5 | 1 | 1 | 0 | lookup | 0 | NULL | | 6 | 1 | 1 | 0 | execute_each_row | 0 | N | +-------------------+-------------------+---------+------+------------------+-----------+-----------+ 3 rows in set (0.00 sec) mysql>
3.這都比較常見，如果我們想要獲取數據的前30%呢？
在SQLServer中，我們可以直接使用top 30 percent 就可以了，但在MySQL中，我們得另想辦法
前10%的數據，我們得知道數據一共有多少，那我們就知道該獲取多少條記錄了，然后還得知道記錄的行號。
(1).獲取行號
在MySQL中，獲取行號，要使用變量（注：由於表中的數據比較多，此處我只拿10條記錄來做實驗）
select a.*, @row_num := @row_num + 1 as row_num from kettle.r_step_attribute a, (select @row_num := 0) b order by ID_STEP_ATTRIBUTE limit 10;
+-------------------+-------------------+---------+------+------------------------+-----------+------------------------------------------------------+---------+ | ID_STEP_ATTRIBUTE | ID_TRANSFORMATION | ID_STEP | NR | CODE | VALUE_NUM | VALUE_STR | row_num | +-------------------+-------------------+---------+------+------------------------+-----------+------------------------------------------------------+---------+ | 1 | 1 | 1 | 0 | PARTITIONING_SCHEMA | 0 | NULL | 1 | | 2 | 1 | 1 | 0 | PARTITIONING_METHOD | 0 | none | 2 | | 3 | 1 | 1 | 0 | sql | 0 | SELECT <values> FROM <table name> WHERE <conditions> | 3 | | 4 | 1 | 1 | 0 | limit | 0 | 0 | 4 | | 5 | 1 | 1 | 0 | lookup | 0 | NULL | 5 | | 6 | 1 | 1 | 0 | execute_each_row | 0 | N | 6 | | 7 | 1 | 1 | 0 | variables_active | 0 | N | 7 | | 8 | 1 | 1 | 0 | lazy_conversion_active | 0 | N | 8 | | 9 | 1 | 1 | 0 | cached_row_meta_active | 0 | N | 9 | | 10 | 1 | 1 | 0 | cluster_schema | 0 | NULL | 10 | +-------------------+-------------------+---------+------+------------------------+-----------+------------------------------------------------------+---------+ 10 rows in set (0.00 sec) mysql>
(2).前30%
有了行號，我們就可以根據行號和總記錄數來過濾了
select * from (select a.*, @row_num := @row_num + 1 as row_num from kettle.r_step_attribute a, (select @row_num := 0) b order by ID_STEP_ATTRIBUTE limit 10) mm where mm.row_num <= (@row_num * 0.3);
+-------------------+-------------------+---------+------+---------------------+-----------+------------------------------------------------------+---------+ | ID_STEP_ATTRIBUTE | ID_TRANSFORMATION | ID_STEP | NR | CODE | VALUE_NUM | VALUE_STR | row_num | +-------------------+-------------------+---------+------+---------------------+-----------+------------------------------------------------------+---------+ | 1 | 1 | 1 | 0 | PARTITIONING_SCHEMA | 0 | NULL | 1 | | 2 | 1 | 1 | 0 | PARTITIONING_METHOD | 0 | none | 2 | | 3 | 1 | 1 | 0 | sql | 0 | SELECT <values> FROM <table name> WHERE <conditions> | 3 | +-------------------+-------------------+---------+------+---------------------+-----------+------------------------------------------------------+---------+ 3 rows in set (0.00 sec) mysql>
@row_num這個變量，是按行來的，所以我們再where條件中使用的時候，他是最后1條記錄的行號，及總記錄數
總記錄數乘以30%，就是前30%的記錄數，這樣，我們就獲取了前X%數據
參考鏈接：https://yuguiyang.github.io/2017/10/29/mysql-handbook-15/