I have a simple table:
CREATE TABLE [dbo].[Word] (
[WordId] INT IDENTITY (1, 1) NOT NULL,
[NameId] INT NOT NULL
PRIMARY KEY CLUSTERED ([WordId] ASC)
);
I have a unique index on NameId
I am trying to update this table and change one column to a random number:
UPDATE Word SET NameId = ROUND(RAND()*2147483647,0)
I realize there is a very very small chance this will not work but it's actually failing every time even though the table has only a very small number of rows the update always fails and says there's a duplicate.
Can anyone tell me what's happening here and also suggest a way to update this table so that there's no duplicate values of NameId created most of the time.
You are updating every NameId with same value, use WHERE statement to update only one row
EDIT: This should do the trick you are looking for, NewId() generates new id for each row
UPDATE Word SET NameId = abs(checksum(NewId()) % 2147483647)
How many rows do you have? It could be a case of the birthday paradox.
Have you tried doing this:
SELECT ROUND(RAND()*2147483647,0) FROM Word
Do the numbers really need to be pseudo-random? You could use row_number() to make them increment.
In a database application, I want to insert, update and delete records in a table of database.
Table is as below:
In this table, Ga1_ID is Primary Key.
Suppose, I insert 5 records as show currently.
In second attempt, if I want to insert 5 other records and if any of these new records contains a primary key attribute which is already present in table it show error. Its fine.
But, when I insert new 5 records... how I can verify these new records's primary key value is not present. I mean, how to match or calculate the already present primary key attributes and then insert new records.
What is the best approach to manage this sort of situation ?
use following query in dataadapter:
da=new SqlDataAdapter("select Ga1_ID from table where Ga1_ID=#pkVal",conn);
DataSet=new DataSet();
da.fill(ds);
//pass parameter for #pkVal
da.SelectCommand.Parameters(1).Value = pkValue;
if(ds.Tables[0].Rows.Count>0) //If number of rows >0 then record exists
BEGIN
messagebox.show("Primary key present");
END
Hope its helpful.
Do not check existing records in advance, i.e. do not SELECT and then INSERT. A better (and pretty common) approach is to try to INSERT and handle exceptions, in particular, catch a primary key violation if any and handle it.
Do the insert in a try/catch block, with different handling in case of a primary key violation exception and other sql exception types.
If there was no exception, then job's done, record was inserted.
If you caught a primary key violation exception, then handle it appropriately (your post does not specify what you want to do in this case, and it's completely up to you)
If you want to perform 5 inserts at once and want to make sure they all succeed or else roll back if any of them failed, then do the inserts within a transaction.
you can do a lookup first before inserting.
IF EXISTS (SELECT * FROM tableName WHERE GA1_id=#newId)
BEGIN
UPDATE tableName SET Ga1_docid = #newdocID, GA1_fieldNAme = #newName, Ga1_fieldValue = #newVal where GA1_id=#newId
END
ELSE
BEGIN
INSERT INTO tableName(GA1_ID, Ga1_docid, GA1_fieldNAme Ga1_fieldValue) VALUES (value1,val2,value3,value4)
END
If you're using SQL Server 2012, use a sequence object - CREATE SEQUENCE.
This way you can get the next value using NEXT VALUE FOR.
With an older SQL Server version, you need to create the primary key field as an IDENTITY field and use the SCOPE_IDENTITY function to get the last identity value and then increment it manually.
Normally, you would like to have a surrogate key wich is generally an identity column that will automatically increment when you are inserting rows so that you don't have to care about knowing which id already exists.
However, if you have to manually insert the id there's a few alternatives for that and knowing wich SQL database you are using would help, but in most SQL implementations, you should be able to do something like:
IF NOT EXISTS
IF NOT EXISTS(
SELECT 1
FROM your_table
WHERE Ga1_ID = 1
)
INSERT INTO ...
SELECT WHERE NOT EXISTS
INSERT INTO your_table (col_1, col_2)
SELECT col_1, col_2
FROM (
SELECT 1 AS col_1, 2 AS col_2
UNION ALL
SELECT 3, 4
) q
WHERE NOT EXISTS (
SELECT 1
FROM your_table
WHERE col_1 = q.col_1
)
For MS SQL Server, you can also look at the MERGE statement and for MySQL, you can use the INSERT IGNORE statement.
I'm trying to formulate some check constraints in SQL Anywhere 9.0.
Basically I have schema like this:
CREATE TABLE limits (
id INT IDENTITY PRIMARY KEY,
count INT NOT NULL
);
CREATE TABLE sum (
user INT,
limit INT,
my_number INT NOT NULL CHECK(my_number > 0),
PRIMARY KEY (user, limit)
);
I'm trying to force a constraint my_number for each limit to be at most count in table.
I've tried
CHECK ((SELECT sum(my_number) FROM sum WHERE limit = limit) <= (SELECT count FROM limits WHERE id = limit))
and
CHECK (((SELECT sum(my_number) FROM sum WHERE limit = limit) + my_number) <= (SELECT count FROM limits WHERE id = limit))
and they both seem not to do the correct thing. They are both off by one (meaning once you get a negative number, then insertion will fail, but not before that.
So my question is, with what version of the table are these subqueries being executed against? Is it the table before the insertion happens, or does the subquery check for consistency after the insert happens, and rolls back if it finds it invalid?
I do not really understand what you try to enforce here but based on this help topic.
Using CHECK constraints on columns
Once a CHECK condition is in place, future values are evaluated
against the condition before a row is modified.
I would go for a before insert trigger. You have more options and can bring up a better error message.
The table I am working with does not have a standard auto-increment field to use as a primary key, so I need to come up with a way to automatically calculate the value that should be used in the field.
My first thought was to create a trigger to happen AFTER INSERT, however, as far as I can tell, there's no easy way to reference the row that was just inserted. I could do something like
UPDATE `table` SET `reference_number` = (SELECT ....) WHERE `reference_number` IS NULL
but because reference_number is a PRIMARY KEY, it cannot be null. (Does that mean it would be an empty string ''?)
Is there a better way to do this?
CREATE TRIGGER mkuuid BEFORE INSERT ON SomeTable
FOR EACH ROW BEGIN
SET NEW.primary_key = UUID_SHORT();
END
I have a MySQL table with a primary key field that has AUTO_INCREMENT on.
After reading other posts on here I've noticed people with the same problem and with varied answers. Some recommend not using this feature, others state it can't be 'fixed'.
I have:
table: course
fields: courseID, courseName
Example: number of records in the table: 18. If I delete records 16, 17 and 18 - I would expect the next record entered to have the courseID of 16, however it will be 19 because the last entered courseID was 18.
My SQL knowledge isn't amazing but is there anyway to refresh or update this count with a query (or a setting in the phpMyAdmin interface)?
This table will relate to others in a database.
Given all the advice, I have decided to ignore this 'problem'. I will simply delete and add records whilst letting the auto increment do it's job. I guess it doesn't really matter what the number is since it's only being used as a unique identifier and doesn't have a (as mentioned above) business meaning.
For those who I may have confused with my original post: I do not wish to use this field to know how many records I have. I just wanted the database to look neat and have a bit more consistency.
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?
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.
Try :
SET #num := 0;
UPDATE your_table SET id = #num := (#num+1);
ALTER TABLE `your_table` 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
If somewhere on your code something use the autoincremented value... maybe this attribution will cause problem.
If you don't use this value in your code everything should be ok.
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.
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.
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.
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;
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.
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.
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.
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.
You can use your mysql client software/script to specify where the primary key should start from after deleting the required records.
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.
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.
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");
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();
}
}