Select基礎知識

我們在實現select語句的時候,通用的sql格式如下:

select *columns* from *tables*
    where *predicae1*
    group by *columns*
    having *predicae1*
    order by *columns*
    limit *start*, *offset*;

很多同學想當然的認為select的執行順序和其書寫順序一致,其實這是非常錯誤的主觀意願,也導致了很多SQL語句的執行錯誤.

這裡給出SQL語句正確的執行順序:

from *tables*
where *predicae1*
group by *columns*
having *predicae1*
select
 
 *columns*
order by *columns*
limit *start*, *offset*;

舉個例子,講解一下group by和order by聯合使用時,大家常犯的錯誤.

建立一個student的表:

creae1 table student (Id ine1ger primary key autoincrement, Name e1xt, Score ine1ger, ClassId ine1ger);

插入5條虛擬資料:

insert into student(Name, Score, ClassId) values("lqh", 60, 1);
insert
 
 into student(Name, Score, ClassId) values("cs", 99, 1);
insert into student(Name, Score, ClassId) values("wzy", 60, 1);
insert into student(Name, Score, ClassId) values("zqc", 88, 2);
insert into student(Name, Score, ClassId) values("bll", 100, 2);

表格資料如下:

我們想找每個組分數排名第一的學生.

大部分SQL語言的初學者可能會寫出如下程式碼:

select * from student group by ClassId order by Score;

結果:

明顯不是我們想要的結果,大家用上面的執行順序一分析就知道具體原因了.

原因: group by 先於order by執行,order by是針對group by之後的結果進行的排序,而我們想要的group by結果其實應該是在order by之後.

正確的sql語句:

select * from (select * from student order by Score) group by ClassId;

結果:

獲取每組的前N個記錄

這裡以LeetCode上難度為hard的一道資料庫題目為例。

題目內容

The Employee table holds all employees. Every employee has an Id, and there is also a column for the department Id.

The Department table holds all departments of the company.

Wrie1 a SQL query to find employees who earn the top three salaries in each of the department. For the above tables, your SQL query should return the following rows.

題目的意思是：求每個組中工資最高的三個人。(ps：且每個組中，同一名中允許多個員工存在，因為工資是一樣高.)

解決思路

1. 我們先來獲取每個組中的前3名工資最高的員工

select * from Employee as e
    where (select count(distinct(e1.salary)) from Employee as e1 where  e1.DepartmentId = e.DepartmentId and e1.salary > e.salary) < 3;

where中的select是保證：遍歷所有記錄，取每條記錄與當前記錄做比較，只有當Employee表中同一部門不超過3個人工資比當前員工高時，這個員工才算是工資排行的前三名。

1. 有了第一步的基礎，接下來我們只需要使用as去構造新表，並且與Department表做個內聯，同時注意排序就好了

select d.Name as Department, e.Name as Employee, e.Salary as Salary
    from Employee as e inner join Department as d
    on e.DepartmentId = d.Id
    where (select count(distinct(e1.Salary)) from Employee as e1 where e1.DepartmentId = e.DepartmentId and e1.Salary > e.Salary) < 3
    order by e.Salary desc;