Zero foundation self-study SQL course | syntax sequence and execution sequence of SQL statements

Hello everyone , I'm Ning Yi .

Today's talk SQL Course No 17 course ：SQL Statement writing order and execution order .

SQL The writing order of statements is not consistent with the execution order , Let's make it clear through this lesson .

1、 Writing order

The order in which statements are written is important , If the order is wrong, it will be reported as wrong .

We have learned from the previous course SELECT、WHERE、ORDER BY、LIMIT、JOIN、UNION、GROUP BY、HAVING Sentence usage .

If these statements SQL At the same time , The writing order is like this ：

SELECT < Field name > 
FROM < Table name >
JOIN < Table name > 
ON < Connection condition >
WHERE < filter >
GROUP BY < Field name >
HAVING < filter >
UNION
ORDER BY < Field name >
LIMIT < Limit the number of rows >;

Looks complicated , But we don't have to memorize it , Do more practice , It's natural to remember .

2、 Execution order

1.FORM： choice from Following table , Generate virtual tables 1.

2.ON：ON yes JOIN The connection condition of , Rows that meet the join criteria are recorded in the virtual table 2 in .

3.JOIN： If you specify LEFT JOIN, The unmatched rows in the reserved table will be added to the virtual table as external rows 2 in , Generate virtual tables 3. If there are more than one JOIN link , Will repeat step 1~3, Until all tables are processed .

4.WHERE： For virtual tables 3 Conduct WHERE filter , Qualified records will be inserted into the virtual table 4 in .

5.GROUP BY： according to GROUP BY Column in clause , For virtual tables 2 Group the records in , Generate virtual tables 5.

6.HAVING： For virtual tables 5 Conduct HAVING Filter , Qualified records will be inserted into the virtual table 6 in .

7.SELECT：SELECT Do not execute until the next step , Select the specified column , Insert into virtual table 7 in .

8.UNION：UNION Two connected SELECT Query statement , Will repeat step 1~7, Generate two virtual tables 7,UNION These records will be merged into the virtual table 8 in .

9.ORDER BY: Put the virtual table 8 Sort the records in , Virtual table 9.

10.LIMIT： Take out the record of the specified line , Return result set .

3、 summary ：

Writing order ：SELECT -> FROM -> JOIN -> ON -> WHERE -> GROUP BY -> HAVING -> UNION -> ORDER BY ->LIMIT

Execution order ：FROM -> ON -> JOIN -> WHERE -> GROUP BY -> HAVING -> SELECT -> UNION -> ORDER BY ->LIMIT

4、 What to pay attention to ：

1.SELECT Statements are always written at the beginning , But execute after most statements . So in SQL In the sentence , We can't WHERE、GROUP BY、 HAVING Statement used in SELECT Alias set in .

however MYSQL There is a characteristic , stay GROUP BY、 HAVING In the sentence , have access to SELECT Alias set in . It's not because MYSQL Will be implemented in advance SELECT, It's because GROUP BY This step returns the cursor , Just have a look at it .

2. Whatever the writing order , Or execution sequence ,UNION Are all in line ORDER BY Ahead .SQL Statement will put all UNION After segment merging , And sort it out .

We have summarized in this lesson sql The order in which statements are written and executed , It helps us to understand these sql Statement optimization , Improve our database understanding . Build our back to write complex SQL The basis of .

If you can master it SQL Statement writing order and execution order , So congratulations , It can solve most of the problems SQL Problem. .

In the next class, we will start to talk about subquery , I understand the content of this lesson , Write more complex later SQL It's handy ~

Click on Focus on , Update the course and notify the first time ~