Interview shock 58: Six differences among truncate, delete and drop!

author | Lei brother

source | Java Analysis of the real interview questions （ID：aimianshi666）

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stay MySQL   in , Use truncate、delete and drop Can be deleted , But they 3 The usage scenarios and execution effects of are completely different , Next, let's take an inventory .

truncate、delete、drop Overview of differences

they 3 The differences between the two are shown in the following table ：

Difference point	drop	truncate	delete
Execution speed	fast	Faster	slow
Order classification	DDL（ Data definition language ）	DDL（ Data definition language ）	DML（ Data operation language ）
Delete object	Delete the entire table and table structure , And the index of the table 、 Constraints and triggers .	Delete only table data , The structure of the table 、 Indexes 、 Constraints, etc. are preserved .	Delete only all or part of the table data , Table structure 、 Indexes 、 Constraints, etc. are preserved .
Delete the condition (where)	Out-of-service	Out-of-service	You can use
Roll back	Cannot roll back	Cannot roll back	Roll back
Self increasing initial value	-	Reset	No reset

Next, we use cases to demonstrate the difference between them .

preparation

Before the official start , Let's create a user table and user test data , It is convenient for subsequent demonstration ：

CREATE TABLE `userinfo` (
  `id` int(11) NOT NULL AUTO_INCREMENT comment ' Number ',
  `name` varchar(250) NOT NULL comment ' full name ' unique,
  `balance` decimal(10,2) NOT NULL DEFAULT '0.00' comment ' The account balance ',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
insert into userinfo values(1,' Zhang San ',1000),(2,' Li Si ',500),(3,' Wang Wu ',2000),(4,' Li Liu ',500);

The table structure and data created are shown in the following figure ：

1. Deleting objects is different

delete and truncate Delete only table data , Do not delete table structure , among delete The result after deletion is as follows ： Let's restore the table to its original state , Reuse truncate Delete , The execution result is shown in the figure below ： Restore the table to its original state , perform drop Delete statements , The execution result is shown in the figure below ： As can be seen from the above results ,delete and truncate Delete only table data , and drop Delete the table structure and table data .

2. Deletion conditions support different

truncate and drop Adding... Is not supported where Conditions , and delete Support where Conditions , As shown in the figure below ：

3. Command categories are different

truncate、delete and drop Belongs to SQL Different categories ,SQL Divided into the following 3 class ：

1. DDL【Data Definition Language】 Data definition language , The structure used to maintain stored data represents instructions : create、drop、alter、truncate.

2. DML【Data Manipulation Language】 Data manipulation language , It is used to operate on data ：insert,delete,update,DML There is another one in the DQL, Data query language , Delegate instruction is select.

3. DCL【Data Control Language】 Data control language , Mainly responsible for authority management and affairs representative instructions ：grant,revoke,commit.

among delete Belong to DML, and truncate and drop Belong to DDL.

PS：truncate Copy a new table structure first , Delete the old table structure and data together , So it belongs to the data definition language DDL, Not a data manipulation language DML.

4. Rollback support is different

delete Belong to DML Support transaction rollback , and truncate and drop Belong to DDL, It will take effect immediately after implementation , And the data is unrecoverable , Now let's verify . First of all MySQL Automatic transaction commit close for , The default value for automatic transaction commit is “ON” That is, automatic submission is enabled , As shown in the figure below ： We use the following command to automatically submit （ Business ） Turn off the ：

set autocommit=off;

Query the settings of automatic transaction submission again. The results are as follows ： Now let's demonstrate delete Rollback operation for , As shown in the figure below ： As can be seen from the above results delete Then you can restore （ Roll back ） Of , and truncate and drop After that, it cannot be rolled back , You old iron can use the same method to test the latter two SQL Implementation .

5. Autoincrement initialization is different

delete The initial value of the auto increment field will not be reset , As shown in the figure below ： and truncate Will reset the initial value of the auto increment field , As shown in the figure below ：

6. Different execution speeds

delete It's line by line , And the operation log will be recorded during execution , For future rollback , therefore delete The execution speed of is relatively slow ; and truncate First, copy a new table structure , Delete the original table as a whole , So its execution speed is in the middle , and drop The fastest execution speed .

summary

truncate、drop and delete The main differences are as follows 6 spot ：

1. Execution speed ：drop > truncate > detele.

2. delete and truncate Delete only table data , and drop The table data, table structure and table index will be deleted 、 Constraints and triggers .

3. delete You can add where The condition enables partial data deletion , and truncate and drop Cannot add where The condition is to delete as a whole .

4. truncate and drop It's immediate execution , And it can't be recovered ; and delete I can walk away from business , Can be withdrawn and restored .

5. truncate Will reset the auto increment column to 1, and delete Autoincrement columns are not reset .

6. truncate and drop yes DDL sentence , and delete yes DML sentence .

It's up to you to judge right and wrong , Disdain is to listen to people , Gain or loss is more important than number .

official account ：Java Analysis of the real interview questions

Interview collection ：https://gitee.com/mydb/interview

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