SQL learning notes (I)

（ This article is being updated ）

Some great advanced tutorials

1. SQL Advanced techniques https://zhuanlan.zhihu.com/p/139856374
   Include content ：SQL The writing standard of 、SQL Some advanced use skills of （ Skillfully use CASE WHEN Make statistics & to update 、 Skillfully use HAVING Clause 、 Self join 、 Skillfully use COALESCE function ）、SQL Performance optimization skills .
2. mysql Video combat tutorial online learning - database - Frog lesson net https://www.wkcto.com/courses.html?list=mysql

Some points to think about when brushing questions

I want to master it recently SQL, But undergraduate teachers teach relatively shallow , And most of them have been forgotten , So I finished it in one day SQL Basic tutorials for , Here is an easy-to-use self-study SQL Website （ It can run in real time ）： self-taught SQL network . At the same time, I also did a little practice LeetCode
This article will record the important points in the learning process , For future study reference .

1. Pay attention to the writing order . The typical correct sequence is as follows ：

   SELECT DISTINCT column, AGG_FUNC(column_or_expression)
   FROM mytable
       JOIN another_table
         ON mytable.column = another_table.column
       WHERE constraint_expression
       GROUP BY column
       HAVING constraint_expression
       ORDER BY column ASC/DESC
       LIMIT count OFFSET COUNT;
   

2. If you don't have to GROUP BY grammar , ordinary WHERE That's enough. .

3. If you want to separate statistics according to two dimensions , stay GROUP BY Just put two inside . for example ：

   select role,count(*),(case when Building is not null then 1 else 0 end) bn
   from employees group by role,bn
   

4. Query execution order ：FROM and JOIN;WHERE;GROUP BY;HAVING;SELECT;DISTINCT
   If there are duplicate data rows DISTINCT Will be responsible for weight removal .

5. ORDER BY
   When the result set is determined ,ORDER BY Sort the results . because SELECT The expression in has been executed . You can use AS Alias .

6. LIMIT / OFFSET
   Last LIMIT and OFFSET Intercept some data from the sorting results .

7. When selecting the highest data , If you need to display a null value , have access to IFNULL(...,NULL)

8. LIMIT The parameter of cannot be a formula , If you need to change a value, you can use the statement SET N = N-1; If you need to update the table , Can cooperate with UPDATE Use . for example ：

   UPDATE Salaries
   SET salary = CASE WHEN salary >= 10000 THEN salary * 0.9
   WHEN salary < 10000 THEN salary * 1.2
   ELSE salary END;