1.delete

stay MySQL in , The total number of deleted methods is 3 Kind of ：delete、truncate、drop, The usage and usage scenarios of the three are completely different , Let's look at it in detail .

1.delete

detele Can be used to delete some or all of the data in a table , Its usage syntax is as follows ：

delete from table_name [where...] [order by...] [limit...]

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PS：[] The commands in are optional , Can be omitted .

If we want to delete the top math scores in the student list 3 Students , You can use the following SQL：

delete from student order by math desc limit 3;

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1.1 delete Realization principle

stay InnoDB In the engine ,delete The operation does not really delete the data , Instead, the data is marked for deletion , Marked as deleted , We can do this by MySQL Set to non auto submit mode , To test and verify . Settings for non auto submit mode SQL as follows ：

set autocommit=0;

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After that, a data delete Delete the , And then use rollback Rollback operation , Finally, verify whether the data we deleted before still exists , If the data still exists, it means delete The data is not really deleted , It just identifies the data as deleted , verification SQL And execution results are shown in the figure below ：

1.2 About the auto increment column

stay InnoDB In the engine , Used delete After deleting all the data , The auto increment column is not reset to the initial value , We can use the following command to verify ：

2.truncate

truncate Execution effect and delete similar , It is also used to delete all row data in the table , Its usage syntax is as follows ：

truncate [table] table_name

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truncate In use and delete The big difference ,delete You can use conditional expressions to delete some data , and truncate Cannot add conditional expression , So it can only delete all row data , For example, the following truncate Added where An error will be reported after the command ：

2.1 truncate Realization principle

truncate It seems that only row data has been deleted , But it is DDL sentence , That is to say Data Definition Language Data definition language , It is used to maintain the structure instructions of stored data , So this is also the same as delete Commands are different ,delete The statement belongs to DML,Data Manipulation Language Data manipulation language , Used to manipulate data . Why? truncate Only the row data is deleted , Column data was not deleted （ Data such as fields and indexes ） nevertheless DDL Language ？ This is because truncate In essence, a new table structure is created , Delete the original table , So it belongs to DDL Language , Instead of DML Language .

2.2 Reset auto increment column

truncate stay InnoDB The auto incrementing column will be reset in the engine , The following command shows ：

3.drop

drop Unlike the previous two commands, which only delete the row data of the table ,drop The row data of the entire table and the table structure will be deleted , Its syntax is as follows ：

DROP [TEMPORARY] TABLE [IF EXISTS] tbl_name [,tbl_name]

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among TEMPORARY It means temporary table , In general, this command will be ignored .

drop An example is as follows ：

Differences among the three

1. Data recovery ：delete Can recover deleted data , and truncate and drop Cannot recover deleted data .
2. In terms of execution speed ：drop > truncate > delete.
3. Delete data ：drop Delete the entire table , Contains row data and fields 、 Index and other data , and truncate and drop Only row data is deleted .
4. Add conditional aspect ：delete have access to where Expression to add query criteria , and truncate and drop Can not add where Query criteria .
5. Reset auto incrementing aspect ： stay InnoDB In the engine ,truncate Auto incrementing columns can be reset , and delete Auto incrementing columns cannot be reset .

summary

delete、truncate Can be used to delete row data in a table , and drop Is to delete the whole table , The deleted data contains all row data and fields 、 Index and other data , among delete Deleted data can be recovered , and truncate and drop Is irrecoverable , But in terms of execution efficiency , The latter two deletion methods have great advantages , Therefore, select the corresponding delete command according to the actual scene , Of course truncate and drop You should also be careful when using the deletion method of these unrecoverable data .

It's up to you to judge right and wrong , Disdain is to listen to people , Gain or loss is more important than number . Interview collection ：https://gitee.com/mydb/interview