Summary: in this tutorial, we will show you how to use MySQL sequence to automatically generate unique numbers for ID columns of tables.

• Trigger Primary Key Generation On Update Mysql Free
• Mysql Show Primary Key
• Key Generator

Creating MySQL sequence

MySQL UUID vs. Auto-Increment INT as primary key Pros. Using UUID for a primary key brings the following advantages: UUID values are unique across tables, databases, and even servers that allow you to merge rows from different databases or distribute databases across servers.

In MySQL, a sequence is a list of integers generated in the ascending order i.e., 1,2,3… Many applications need sequences to generate unique numbers mainly for identification e.g., customer ID in CRM, employee numbers in HR, and equipment numbers in the services management system.

1. Jun 19, 2003  you could create a SEQUENCE and then in the trigger initialize the primary key column with the next value from this sequence. Note that using this mechanism, you may get broken sequences in the table (if you rollback the transaction, for example, then still the sequence will be incremented).
2. The syntax to create an AFTER UPDATE Trigger in MySQL is: CREATE TRIGGER triggername AFTER UPDATE ON tablename FOR EACH ROW BEGIN - variable declarations - trigger code END; Parameters or Arguments triggername The name of the trigger to create. AFTER UPDATE It indicates that the trigger will fire after the UPDATE operation is executed.

To create a sequence in MySQL automatically, you set the AUTO_INCREMENT attribute to a column, which typically is a primary key column.

The following rules are applied when you use the AUTO_INCREMENT attribute:

• Each table has only one AUTO_INCREMENT column whose data type is typically the integer.
• The AUTO_INCREMENT column must be indexed, which means it can be either PRIMARY KEY or UNIQUE index.
• The AUTO_INCREMENT column must have a NOT NULL constraint. When you set the AUTO_INCREMENT attribute to a column, MySQL automatically adds the NOT NULL constraint to the column implicitly.

Creating MySQL sequence example

The following statement creates a table named employees that has the emp_no column is an AUTO_INCREMENT column:

How MySQL sequence works

The AUTO_INCREMENT column has the following attributes: Windows 7 home basics product key generator.

• The starting value of an AUTO_INCREMENT column is 1 and it is increased by 1 when you insert a NULLvalue into the column or when you omit its value in the INSERT statement.
• To obtain the last generated sequence number, you use the LAST_INSERT_ID() function. We often use the last insert ID for the subsequent statements e.g., insert data into the tables. The last generated sequence is unique across sessions. In other words, if another connection generates a sequence number, from your connection you can obtain it by using the LAST_INSERT_ID() function.
• If you insert a new row into a table and specify a value for the sequence column, MySQL will insert the sequence number if the sequence number does not exist in the column or issue an error if it already exists. If you insert a new value that is greater than the next sequence number, MySQL will use the new value as the starting sequence number and generate a unique sequence number greater than the current one for the next usage. This creates gaps in the sequence.
• If you use the UPDATE statement to update values in the AUTO_INCREMENT column to a value that already exists, MySQL will issue a duplicate-key error if the column has a unique index. If you update an AUTO_INCREMENT column to a value that is greater than the existing values in the column, MySQL will use the next number of the last insert sequence number for the next row. For example, if the last insert sequence number is 3, you update it to 10, the sequence number for the new row is 4.
• If you use the DELETE statement to delete the last inserted row, MySQL may or may not reuse the deleted sequence number depending on the storage engine of the table. A MyISAM table does not reuse the deleted sequence numbers if you delete a row e.g., the last insert id in the table is 10, if you remove it, MySQL still generates the next sequence number which is 11 for the new row. Similar to MyISAM tables, InnoDB tables do not reuse sequence number when rows are deleted.

Once you set the AUTO_INCREMENT attribute for a column, you can reset the auto increment value in various ways e.g., by using the ALTER TABLE statement.

Let’s take a look at some example to get a better understanding of the MySQL sequence.

First, insert two new rows into the employees table:

Second, select data from the employees table:

Third, delete the second employee whose emp_no is 2:

Fourth, insert a new employee:

Because the storage engine of the employees table is InnoDB, it does not reuse the deleted sequence number. The new row has emp_no 3.

Fifth, update an existing employee with emp_no 3 to 1:

MySQL issued an error of duplicate entry for the primary key. Let’s fix it.

Trigger Primary Key Generation On Update Mysql Free

Sixth, insert a new employee after updating the sequence number to 10:

Mysql Show Primary Key

The next sequence number of the last insert is number 4, therefore, MySQL uses number 4 for the new row instead of 11.

Key Generator

In this tutorial, you have learned how to use MySQL sequence to generate unique numbers for a primary key column by assigning the AUTO_INCREMENT attribute to the column.