在 MySQL 中删除后自动增量

you can select the ids like so:

set @rank = 0;
select id, @rank:=@rank+1 from tbl order by id

the result is a list of ids, and their positions in the sequence.

you can also reset the ids like so:

set @rank = 0;
update tbl a join (select id, @rank:=@rank+1 as rank from tbl order by id) b
on a.id = b.id set a.id = b.rank;

you could also just print out the first unused id like so:

select min(id) as next_id from ((select a.id from (select 1 as id) a
left join tbl b on a.id = b.id where b.id is null) union
(select min(a.id) + 1 as id from tbl a left join tbl b on a.id+1 = b.id
where b.id is null)) c;

after each insert, you can reset the auto_increment:

alter table tbl auto_increment = 16

or explicitly set the id value when doing the insert:

insert into tbl values (16, 'something');

typically this isn't necessary, you have count(*) and the ability to create a ranking number in your result sets. a typical ranking might be:

set @rank = 0;
select a.name, a.amount, b.rank from cust a,
(select amount, @rank:=@rank+1 as rank from cust order by amount desc) b
where a.amount = b.amount

customers ranked by amount spent.

ALTER TABLE foo AUTO_INCREMENT=1

If you've deleted the most recent entries, that should set it to use the next lowest available one. As in, as long as there's no 19 already, deleting 16-18 will reset the autoincrement to use 16.

EDIT: I missed the bit about phpmyadmin. You can set it there, too. Go to the table screen, and click the operations tab. There's an AUTOINCREMENT field there that you can set to whatever you need manually.

Primary autoincrement keys in database are used to uniquely identify a given row and shouldn't be given any business meaning. So leave the primary key as is and add another column called for example courseOrder. Then when you delete a record from the database you may want to send an additional UPDATE statement in order to decrement the courseOrder column of all rows that have courseOrder greater than the one you are currently deleting.

As a side note you should never modify the value of a primary key in a relational database because there could be other tables that reference it as a foreign key and modifying it might violate referential constraints.

You can use your mysql client software/script to specify where the primary key should start from after deleting the required records.

You shouldn't be relying on the AUTO_INCREMENT id to tell you how many records you have in the table. You should be using SELECT COUNT(*) FROM course. ID's are there to uniquely identifiy the course and can be used as references in other tables, so you shouldn't repeat ids and shouldn't be seeking to reset the auto increment field.

What you're trying to do sounds dangerous, as that's not the intended use of AUTO_INCREMENT.

If you really want to find the lowest unused key value, don't use AUTO_INCREMENT at all, and manage your keys manually. However, this is NOT a recommended practice.

Take a step back and ask "why you need to recycle key values?" Do unsigned INT (or BIGINT) not provide a large enough key space?

Are you really going to have more than 18,446,744,073,709,551,615 unique records over the course of your application's lifetime?

I got a very simple but tricky method.

While deleting a row, you can preserve the IDs into another temporary table. After that, when you will insert new data into the main table then you can search and pick IDs from the temporary table. So use a checking here. If the temporary table has no IDs then calculate maximum ID into the main table and set the new ID as: new_ID = old_max_ID+1.

NB: You can not use auto-increment feature here.

I came here looking for an answer to the Title question "MySQL - Auto Increment after delete" but I could only find an answer for that in the questions

• How to delete certain row from mysql table?
• How to reset AUTO_INCREMENT in MySQL?

By using something like:

DELETE FROM table;
ALTER TABLE table AUTO_INCREMENT = 1;

Note that Darin Dimitrov's answer explain really well AUTO_INCREMENT and it's usage. Take a look there before doing something you might regret.

PS: The question itself is more "Why you need to recycle key values?" and Dolph's answer cover that.

Try :

SET @num := 0;

UPDATE your_table SET id = @num := (@num+1);

ALTER TABLE tableName AUTO_INCREMENT = 1;

That'll reset the autoincremented value, and then count every row while a new value is created for it.

example : before

• 1 : first value here
• 2 : second value here
• X : deleted value
• 4 : The rest of the table
• 5 : The rest of the rest..

so the table will display the array : 1,2,4,5

Example : AFTER (if you use this command you will obtain)

• 1 : first value here
• 2 : second value here
• 3 : The rest of the table
• 4 : the rest of the rest

No trace of the deleted value, and the rest of the incremented continues with this new count.

BUT

1. If somewhere on your code something use the autoincremented value... maybe this attribution will cause problem.
2. If you don't use this value in your code everything should be ok.

What you are trying to do is very dangerous. Think about this carefully. There is a very good reason for the default behaviour of auto increment.

Consider this:

A record is deleted in one table that has a relationship with another table. The corresponding record in the second table cannot be deleted for auditing reasons. This record becomes orphaned from the first table. If a new record is inserted into the first table, and a sequential primary key is used, this record is now linked to the orphan. Obviously, this is bad. By using an auto incremented PK, an id that has never been used before is always guaranteed. This means that orphans remain orphans, which is correct.

I can think of plenty of scenarios where you might need to do this, particularly during a migration or development process. For instance, I just now had to create a new table by cross-joining two existing tables (as part of a complex set-up process), and then I needed to add a primary key after the event. You can drop the existing primary key column, and then do this.

ALTER TABLE my_table ADD `ID` INT NOT NULL AUTO_INCREMENT FIRST, ADD PRIMARY KEY (`ID`);

For a live system, it is not a good idea, and especially if there are other tables with foreign keys pointing to it.

There is actually a way to fix that. First you delete the auto_incremented primary key column, and then you add it again, like this:

ALTER TABLE table_name DROP column_name;
ALTER TABLE table_name ADD column_name int not null auto_increment primary key first;

if($id == 1){ // deleting first row
mysqli_query($db,"UPDATE employees  SET id=id-1 WHERE id>1");
}
else if($id>1 && $id<$num){ // deleting middle row
mysqli_query($db,"UPDATE employees  SET id=id-1 WHERE id>$id");
}
else if($id == $num){ // deleting last row
mysqli_query($db,"ALTER TABLE employees AUTO_INCREMENT = $num");
}
else{
echo "ERROR";
}


mysqli_query($db,"ALTER TABLE employees AUTO_INCREMENT = $num");

You may think about making a trigger after delete so you can update the value of autoincrement and the ID value of all rows that does not look like what you wanted to see.

So you can work with the same table and the auto increment will be fixed automaticaly whenever you delete a row the trigger will fix it.

Its definitely not recommendable. If you have a large database with multiple tables, you may probably have saved a userid as id in table 2. if you rearrange table 1 then probably the intended userid will not end up being the intended table 2 id.

here is a function that fix your problem

    public static void fixID(Connection conn, String table) {


try {
Statement myStmt = conn.createStatement();
ResultSet myRs;
int i = 1, id = 1, n = 0;
boolean b;
String sql;


myRs = myStmt.executeQuery("select max(id) from " + table);
if (myRs.next()) {
n = myRs.getInt(1);
}
while (i <= n) {
b = false;
myRs = null;
while (!b) {
myRs = myStmt.executeQuery("select id from " + table + " where id=" + id);
if (!myRs.next()) {
id++;
} else {
b = true;
}
}


sql = "UPDATE " + table + " set id =" + i + " WHERE id=" + id;
myStmt.execute(sql);
i++;
id++;
}


} catch (SQLException e) {
e.printStackTrace();
}
}

MYSQL Query Auto Increment Solution. It works perfect when you have inserted many records during testing phase of software. Now you want to launch your application live to your client and You want to start auto increment from 1.

To avoid any unwanted problems, for safer side First export .sql file.
Then follow the below steps:

• Step 1) First Create the copy of an existing table MySQL Command to create Copy:

  CREATE TABLE new_Table_Name  SELECT * FROM existing_Table_Name;
  

  The exact copy of a table is created with all rows except Constraints.
  It doesn’t copy constraints like Auto Increment and Primary Key into new_Table_name

• Step 2) Delete All rows If Data is not inserted in testing phase and it is not useful. If Data is important then directly go to Step 3.

  DELETE from new_Table_Name;
  

• Step 3) To Add Constraints, Goto Structure of a table

  • 3A) Add primary key constraint from More option (If You Require).
  • 3B) Add Auto Increment constraint from Change option. For this set Defined value as None.
  • 3C) Delete existing_Table_Name and
  • 3D) rename new_Table_Name to existing_Table_Name.

Now It will work perfectly. The new first record will take first value in Auto Increment column.

Here is a step to solve your problem.

On your .php file, just add this query given below:

<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "";


$conn = new mysqli($servername, $username, $password, $dbname);


if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}


//write the number or id you want to start with the next user in AUTO_INCREMENT
$sql = "ALTER TABLE `table_name` AUTO_INCREMENT = number";
$conn->query($sql);
?>

I hope your problem will be solved.