1.在MySQL中，我们要查看前3条记录的话，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.简单的分页也可以使用力limit来完成
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.这都比较常见，如果我们想要获取数据的前30%呢？
在SQLServer中，我们可以直接使用top 30 percent 就可以了，但在MySQL中，我们得另想办法
前10%的数据，我们得知道数据一共有多少，那我们就知道该获取多少条记录了，然后还得知道记录的行号。
(1).获取行号
在MySQL中，获取行号，要使用变量（注：由于表中的数据比较多，此处我只拿10条记录来做实验）
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%
有了行号，我们就可以根据行号和总记录数来过滤了
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这个变量，是按行来的，所以我们再where条件中使用的时候，他是最后1条记录的行号，及总记录数
总记录数乘以30%，就是前30%的记录数，这样，我们就获取了前X%数据
参考链接：https://yuguiyang.github.io/2017/10/29/mysql-handbook-15/
