SQL限制結果達到前50％

Question

我嘗試了兩種方法，但在mysql中失敗。

/*see top 50% students, but this sql can't work*/
select * from student_table order by chinese_score desc limit count(*) * 0.5 ;

/*also can't work*/
set @num= floor((select count(*) from test.student_score)*0.5);
select * from student_table order by chinese_score desc limit @num ;

如何在mysql中解決？

Answer 1

在Mysql中，可以使用用戶定義的變量在單個查詢中完成。

您可以在一個語句中將值存儲在用戶定義的變量中，稍后在另一條語句中引用它。 這使您可以將值從一個語句傳遞到另一個語句。

SELECT * FROM    (
          SELECT student_table.*, @counter := @counter +1 AS counter
          FROM (SELECT @counter:=0) AS initvar, student_table
          ORDER BY student_table.chinese_score DESC
        ) AS result
WHERE counter < (@counter/2) ORDER BY chinese_score DESC;