MySQL statement execution order

Reprinted from ：http://www.cnblogs.com/rollenholt/p/3776923.html

MySQL The statements are divided into 11 Step , As shown in the figure below , The first thing to do is always FROM operation , The last thing to do is LIMIT operation . Each of these operations produces a virtual table , This virtual table serves as a processing input , Only these virtual tables are transparent to users , But only the last virtual table will be returned as a result . If you don't specify a clause in the statement , Then the corresponding steps will be skipped .

Let's analyze each stage of query processing in detail

1. FORM: Yes FROM The table on the left and the table on the right calculate Cartesian product . Produce a virtual table VT1
2. ON: Antithetical table VT1 Conduct ON Screening , Only those matching rows will be recorded in the virtual table VT2 in .
3. JOIN： If you specify OUTER JOIN（ such as left join、 right join）, The unmatched rows in the reserved table will be added to the virtual table as external rows VT2 in , Generate virtual tables VT3, If from Clause contains more than two tables , Then it will be for the last join The result of the connection VT3 Repeat steps with the next table 1~3 These three steps , Until all the watches have been processed .
4. WHERE： For virtual tables VT3 Conduct WHERE filter . Only matching records will be inserted into the virtual table VT4 in .
5. GROUP BY: according to group by Column in clause , Yes VT4 Group the records in , produce VT5.
6. CUBE | ROLLUP: Counter table VT5 Conduct cube perhaps rollup operation , Generating table VT6.
7. HAVING： For virtual tables VT6 application having Filter , Only records that match will be Insert into virtual table VT7 in .
8. SELECT： perform select operation , Select the specified column , Insert into virtual table VT8 in .
9. DISTINCT： Yes VT8 The records in are de duplicated . Generate virtual tables VT9.
10. ORDER BY: Put the virtual table VT9 According to the records in <order_by_list> Sort operations , Generate virtual tables VT10.
11. LIMIT： Take out the record of the specified line , Generate virtual tables VT11, And return the result .