What is the order in which SQL keywords are executed?

The order in which SQL keywords are executed is: first execute the from statement; then the where statement, group by statement; then the having statement; and finally the order by statement.

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sql keyword execution order

Example:

select m.* from(select t.*,rownum r from
(select id,name,e.username,e.realname from departments d ,employees e where d.manager=e.username(+)
order by id desc ) t where rownum<=? ) m where r>?

select m.*(select t.*,t.rownum r from(
select a,b,c,d from table1 order by a desc
) t where rownum < = ? ) m
where r > ?

The standard SQL parsing order is

(1) FROM clause, assemble data from different data sources

(2) WHERE clause, filter records based on specified conditions

(3) GROUP BY clause, combine The data is divided into multiple groups

(4) Use aggregate functions to calculate

(5) Use HAVING clause to filter groups

(6) Calculate all expressions

(7) Use ORDER BY to sort the result set

Example

In the student grade table (tentatively tb_Grade), Group the records with non-empty "candidate name" content according to "candidate name", and filter the grouping results to select the

whose "total score" is greater than 600 points. The SQL statement is:

　select 考生姓名, max(总成绩) as max总成绩
　　from tb_Grade
　　where 考生姓名 is not null
　　group by 考生姓名
　　having max(总成绩) > 600
　　order by max总成绩

In the above example, the execution sequence of the SQL statements is as follows:

(1) First execute the FROM clause and assemble the data from the data source from the tb_Grade table

(2) Execute the WHERE clause, filter all data in the tb_Grade table that is not NULL

(3) Execute the GROUP BY clause, group the tb_Grade table by the "Student Name" column

(4) Calculate the max() aggregate function and find the largest values ​​in the total score according to "total score"

(5) Execute the HAVING clause and filter the courses whose total score is greater than 600 points.

(6) Execute the ORDER BY clause and sort the final results by "Max Score".

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