MySQL TIMESTAMP

 MySQL TIMESTAMP

Summary: in this tutorial, you will learn about MySQL TIMESTAMP data type and  TIMESTAMP column features such as automatic initialization and updating.

Introduction to MySQL TIMESTAMP datatype

The MySQL TIMESTAMP is a temporal data type that holds the combination of date and time. The format of a TIMESTAMP is YYYY-MM-DD HH:MM:SS which is fixed at 19 characters.

The TIMESTAMP value has a range from '1970-01-01 00:00:01' UTC to '2038-01-19 03:14:07' UTC.

When you insert a TIMESTAMP value into a table, MySQL converts it from your connection’s time zone to UTC for storing.

When you query a TIMESTAMP value, MySQL converts the UTC value back to your connection’s time zone. Note that this conversion does not take place for other temporal data types such as DATETIME.

By default, the connection time zone is the MySQL Server’s time zone. And you can use a different time zone when you connect to MySQL Server.

When you retrieve a TIMESTAMP the value that was inserted by a client in a different time zone, you will get a value that is not the same as the value stored in the database. As long as you don’t change the time zone, you can get the same TIMESTAMP value that you stored.

MySQL TIMESTAMP time zone example

Let’s take an example to see how MySQL handles TIMESTAMP values.

First, created a new table named test_timestamp that has a TIMESTAMP column: t1;

CREATE TABLE test_timestamp (
    t1  TIMESTAMP
);

Second, set the session’s time zone to ‘+00:00’ UTC by using the SET time_zone statement.

SET time_zone='+00:00';

Third, insert a TIMESTAMP value into the test_timestamp table.

INSERT INTO test_timestamp(t1)
VALUES('2008-01-01 00:00:01');

Fourth, select the TIMESTAMP value from the test_timestamp table.

SELECT t1 FROM test_timestamp;

Fifth, set the session’s time zone to a different time zone to see what value we will get from the database server:

SET time_zone ='+03:00';

Finally, query data from the table:

SELECT t1 FROM test_timestamp;

As you see, we received a different time value adjusted to the new time zone.

Automatic initialization and updating for TIMESTAMP columns

Consider the following example.

First, creates a table named  categories:

CREATE TABLE categories (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

In the categories table, the created_at column is a TIMESTAMP a column whose default value is set to CURRENT_TIMESTAMP.

Second, inserts a new row into the categories table without specifying the value for the created_at column:

INSERT INTO categories(name) 
VALUES ('A');

SELECT * FROM categories;

As you can see from the output, MySQL used the timestamp at the time of inserting as a default value for the created_at column.

So a TIMESTAMP the column can be automatically initialized to the current timestamp for inserted rows that specify no value for the column. This feature is called automatic initialization.

Third, add a new column named updated_at to the categories table.

ALTER TABLE categories
ADD COLUMN updated_at 
  TIMESTAMP DEFAULT CURRENT_TIMESTAMP 
  ON UPDATE CURRENT_TIMESTAMP;

The default value of the updated_at column is CURRENT_TIMESTAMP.

And, we have a new clause ON UPDATE CURRENT_TIMESTAMP that follows the DEFAULT CURRENT_TIMESTAMP clause. Let’s see its effect.

Fourth, inserts a new row into the categories table.

INSERT INTO categories(name)
VALUES('B');

Fifth, query data from the categories table:

SELECT * FROM categories;

The default value of the column created_at is the timestamp when the row was inserted.

Sixth, update the value in the column name of the row id 2:

UPDATE categories 
SET name = 'B+'
WHERE id = 2;

Seventh, query data from the categories table to check the update:

SELECT *
FROM categories
WHERE id = 2;

Notice that the value in the updated_at column changed to the timestamp at the time the row was updated.

The ability of a TIMESTAMP column to be automatically updated to the current timestamp when the value in any other column in the row changes from its current value is called automatic updating.

The column updated_at is referred to as an auto-updated column.

Note that if you execute the UPDATE statement to update the same value for the name column, the updated_at column will not be updated.

UPDATE categories 
SET name = 'B+'
WHERE id = 2;

The value was in  updated_at remains unchanged.

For more information on automatic initialization and updating, please check out the time initialization on the MySQL website.

As of MySQL 5.6.5, the DATETIME columns also have automatic initialization and updating features. In addition, the DEFAULT_CURRENT_TIMESTAMP and ON UPDATE CURRENT TIMESTAMP can be applied to multiple columns.

In this tutorial, you have learned about MySQL TIMESTAMP data type and how to use automatic initialization and updating features of TIMESTAMP columns.