1、查询“001”课程比“002”课程成绩高的所有学生的学号; select a.S# from (select s#,score from SC where C#='001') a,(select s#,score from SC where C#='002') b where a.score>b.score and a.s#=b.s#; 2、查询平均成绩大于60分的同学的学号和平均成绩; select S#,avg(score) from sc group by S# having
OR、in和union all 查询效率到底哪个快?
网上很多的声音都是说union all 快于 or、in,因为or、in会导致全表扫描,他们给出了很多的实例。
但真的union all真的快于or、in?
EXPLAIN SELECT * from employees where employees.first_NAME ='Georgi' UNION ALL SELECT * from employees where employees.first_NAME ='Bezalel'
这