Migrations are like version control for your database, allowing your team to define and share the application's database schema definition. If you have ever had to tell a teammate to manually add a column to their local database schema after pulling in your changes from source control, you've faced the problem that database migrations solve.
The Laravel Schema facade provides database agnostic support for creating and manipulating tables across all of Laravel's supported database systems. Typically, migrations will use this facade to create and modify database tables and columns.
You may use the make:migration Artisan command to generate a database migration. The new migration will be placed in your database/migrations directory. Each migration filename contains a timestamp that allows Laravel to determine the order of the migrations:
php artisan make:migration create_flights_table
Laravel will use the name of the migration to attempt to guess the name of the table and whether or not the migration will be creating a new table. If Laravel is able to determine the table name from the migration name, Laravel will pre-fill the generated migration file with the specified table. Otherwise, you may simply specify the table in the migration file manually.
If you would like to specify a custom path for the generated migration, you may use the --path option when executing the make:migration command. The given path should be relative to your application's base path.
{tip} Migration stubs may be customized using stub publishing
As you build your application, you may accumulate more and more migrations over time. This can lead to your database/migrations directory becoming bloated with potentially hundreds of migrations. If you would like, you may "squash" your migrations into a single SQL file. To get started, execute the schema:dump command:
php artisan schema:dump
// Dump the current database schema and prune all existing migrations...
php artisan schema:dump --prune
When you execute this command, Laravel will write a "schema" file to your application's database/schema directory. Now, when you attempt to migrate your database and no other migrations have been executed, Laravel will execute the schema file's SQL statements first. After executing the schema file's statements, Laravel will execute any remaining migrations that were not part of the schema dump.
You should commit your database schema file to source control so that other new developers on your team may quickly create your application's initial database structure.
{note} Migration squashing is only available for the MySQL, PostgreSQL, and SQLite databases and utilizes the database's command-line client. Schema dumps may not be restored to in-memory SQLite databases.
A migration class contains two methods: up and down. The up method is used to add new tables, columns, or indexes to your database, while the down method should reverse the operations performed by the up method.
Within both of these methods, you may use the Laravel schema builder to expressively create and modify tables. To learn about all of the methods available on the Schema builder, check out its documentation. For example, the following migration creates a flights table:
<?php
use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;
class CreateFlightsTable extends Migration
{
/**
* Run the migrations.
*
* @return void
*/
public function up()
{
Schema::create('flights', function (Blueprint $table) {
$table->id();
$table->string('name');
$table->string('airline');
$table->timestamps();
});
}
/**
* Reverse the migrations.
*
* @return void
*/
public function down()
{
Schema::drop('flights');
}
}
To run all of your outstanding migrations, execute the migrate Artisan command:
php artisan migrate
If you would like to see which migrations have run thus far, you may use the migrate:status Artisan command:
php artisan migrate:status
Some migration operations are destructive, which means they may cause you to lose data. In order to protect you from running these commands against your production database, you will be prompted for confirmation before the commands are executed. To force the commands to run without a prompt, use the --force flag:
php artisan migrate --force
To roll back the latest migration operation, you may use the rollback Artisan command. This command rolls back the last "batch" of migrations, which may include multiple migration files:
php artisan migrate:rollback
You may roll back a limited number of migrations by providing the step option to the rollback command. For example, the following command will roll back the last five migrations:
php artisan migrate:rollback --step=5
The migrate:reset command will roll back all of your application's migrations:
php artisan migrate:reset
The migrate:refresh command will roll back all of your migrations and then execute the migrate command. This command effectively re-creates your entire database:
php artisan migrate:refresh
// Refresh the database and run all database seeds...
php artisan migrate:refresh --seed
You may roll back and re-migrate a limited number of migrations by providing the step option to the refresh command. For example, the following command will roll back and re-migrate the last five migrations:
php artisan migrate:refresh --step=5
The migrate:fresh command will drop all tables from the database and then execute the migrate command:
php artisan migrate:fresh
php artisan migrate:fresh --seed
{note} The
migrate:freshcommand will drop all database tables regardless of their prefix. This command should be used with caution when developing on a database that is shared with other applications.
To create a new database table, use the create method on the Schema facade. The create method accepts two arguments: the first is the name of the table, while the second is a closure which receives a Blueprint object that may be used to define the new table:
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;
Schema::create('users', function (Blueprint $table) {
$table->id();
$table->string('name');
$table->string('email');
$table->timestamps();
});
When creating the table, you may use any of the schema builder's column methods to define the table's columns.
You may check for the existence of a table or column using the hasTable and hasColumn methods:
if (Schema::hasTable('users')) {
// The "users" table exists...
}
if (Schema::hasColumn('users', 'email')) {
// The "users" table exists and has an "email" column...
}
If you want to perform a schema operation on a database connection that is not your application's default connection, use the connection method:
Schema::connection('sqlite')->create('users', function (Blueprint $table) {
$table->id();
});
In addition, a few other properties and methods may be used to define other aspects of the table's creation. The engine property may be used to specify the table's storage engine when using MySQL:
Schema::create('users', function (Blueprint $table) {
$table->engine = 'InnoDB';
// ...
});
The charset and collation properties may be used to specify the character set and collation for the created table when using MySQL:
Schema::create('users', function (Blueprint $table) {
$table->charset = 'utf8mb4';
$table->collation = 'utf8mb4_unicode_ci';
// ...
});
The temporary method may be used to indicate that the table should be "temporary". Temporary tables are only visible to the current connection's database session and are dropped automatically when the connection is closed:
Schema::create('calculations', function (Blueprint $table) {
$table->temporary();
// ...
});
The table method on the Schema facade may be used to update existing tables. Like the create method, the table method accepts two arguments: the name of the table and a closure that receives a Blueprint instance you may use to add columns or indexes to the table:
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;
Schema::table('users', function (Blueprint $table) {
$table->integer('votes');
});
To rename an existing database table, use the rename method:
use Illuminate\Support\Facades\Schema;
Schema::rename($from, $to);
To drop an existing table, you may use the drop or dropIfExists methods:
Schema::drop('users');
Schema::dropIfExists('users');
Before renaming a table, you should verify that any foreign key constraints on the table have an explicit name in your migration files instead of letting Laravel assign a convention based name. Otherwise, the foreign key constraint name will refer to the old table name.
The table method on the Schema facade may be used to update existing tables. Like the create method, the table method accepts two arguments: the name of the table and a closure that receives an Illuminate\Database\Schema\Blueprint instance you may use to add columns to the table:
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;
Schema::table('users', function (Blueprint $table) {
$table->integer('votes');
});
The schema builder blueprint offers a variety of methods that correspond to the different types of columns you can add to your database tables. Each of the available methods are listed in the table below:
<style> #collection-method-list > p { column-count: 3; -moz-column-count: 3; -webkit-column-count: 3; column-gap: 2em; -moz-column-gap: 2em; -webkit-column-gap: 2em; } #collection-method-list a { display: block; } </style>The bigIncrements method creates an auto-incrementing UNSIGNED BIGINT (primary key) equivalent column:
$table->bigIncrements('id');
The bigInteger method creates a BIGINT equivalent column:
$table->bigInteger('votes');
The binary method creates a BLOB equivalent column:
$table->binary('photo');
The boolean method creates a BOOLEAN equivalent column:
$table->boolean('confirmed');
The char method creates a CHAR equivalent column with of a given length:
$table->char('name', 100);
The dateTimeTz method creates a DATETIME (with timezone) equivalent column with an optional precision (total digits):
$table->dateTimeTz('created_at', $precision = 0);
The dateTime method creates a DATETIME equivalent column with an optional precision (total digits):
$table->dateTime('created_at', $precision = 0);
The date method creates a DATE equivalent column:
$table->date('created_at');
The decimal method creates a DECIMAL equivalent column with the given precision (total digits) and scale (decimal digits):
$table->decimal('amount', $precision = 8, $scale = 2);
The double method creates a DOUBLE equivalent column with the given precision (total digits) and scale (decimal digits):
$table->double('amount', 8, 2);
The enum method creates a ENUM equivalent column with the given valid values:
$table->enum('difficulty', ['easy', 'hard']);
The float method creates a FLOAT equivalent column with the given precision (total digits) and scale (decimal digits):
$table->float('amount', 8, 2);
The foreignId method is an alias of the unsignedBigInteger method:
$table->foreignId('user_id');
The geometryCollection method creates a GEOMETRYCOLLECTION equivalent column:
$table->geometryCollection('positions');
The geometry method creates a GEOMETRY equivalent column:
$table->geometry('positions');
The id method is an alias of the bigIncrements method. By default, the method will create an id column; however, you may pass a column name if you would like to assign a different name to the column:
$table->id();
The increments method creates an auto-incrementing UNSIGNED INTEGER equivalent column as a primary key:
$table->increments('id');
The integer method creates an INTEGER equivalent column:
$table->integer('votes');
The ipAddress method creates an INTEGER equivalent column:
$table->ipAddress('visitor');
The json method creates a JSON equivalent column:
$table->json('options');
The jsonb method creates a JSONB equivalent column:
$table->jsonb('options');
The lineString method creates a LINESTRING equivalent column:
$table->lineString('positions');
The longText method creates a LONGTEXT equivalent column:
$table->longText('description');
The macAddress method creates a column that is intended to hold a MAC address. Some database systems, such as PostgreSQL, have a dedicated column type for this type of data. Other database systems will use a string equivalent column:
$table->macAddress('device');
The mediumIncrements method creates an auto-incrementing UNSIGNED MEDIUMINT equivalent column as a primary key:
$table->mediumIncrements('id');
The mediumInteger method creates a MEDIUMINT equivalent column:
$table->mediumInteger('votes');
The mediumText method creates a MEDIUMTEXT equivalent column:
$table->mediumText('description');
The morphs method is a convenience method that adds a {column}_id UNSIGNED BIGINT equivalent column and a {column}_type VARCHAR equivalent column.
This method is intended to be used when defining the columns necessary for a polymorphic Eloquent relationship. In the following example, taggable_id and taggable_type columns would be created:
$table->morphs('taggable');
The multiLineString method creates a MULTILINESTRING equivalent column:
$table->multiLineString('positions');
The multiPoint method creates a MULTIPOINT equivalent column:
$table->multiPoint('positions');
The multiPolygon method creates a MULTIPOLYGON equivalent column:
$table->multiPolygon('positions');
The method is similar to the timestamps method; however, the column that is created will be "nullable":
$table->nullableTimestamps(0);
The method is similar to the morphs method; however, the columns that are created will be "nullable":
$table->nullableMorphs('taggable');
The method is similar to the uuidMorphs method; however, the columns that are created will be "nullable":
$table->nullableUuidMorphs('taggable');
The point method creates a POINT equivalent column:
$table->point('position');
The polygon method creates a POLYGON equivalent column:
$table->polygon('position');
The rememberToken method creates a nullable, VARCHAR(100) equivalent column that is intended to store the current "remember me" authentication token:
$table->rememberToken();
The set method creates a SET equivalent column with the given list of valid values:
$table->set('flavors', ['strawberry', 'vanilla']);
The smallIncrements method creates an auto-incrementing UNSIGNED SMALLINT equivalent column as a primary key:
$table->smallIncrements('id');
The smallInteger method creates a SMALLINT equivalent column:
$table->smallInteger('votes');
The softDeletesTz method adds a nullable deleted_at TIMESTAMP (with timezone) equivalent column with an optional precision (total digits). This column is intended to store the deleted_at timestamp needed for Eloquent's "soft delete" functionality:
$table->softDeletesTz($column = 'deleted_at', $precision = 0);
The softDeletes method adds a nullable deleted_at TIMESTAMP equivalent column with an optional precision (total digits). This column is intended to store the deleted_at timestamp needed for Eloquent's "soft delete" functionality:
$table->softDeletes($column = 'deleted_at', $precision = 0);
The string method creates a VARCHAR equivalent column of the given length:
$table->string('name', 100);
The text method creates a TEXT equivalent column:
$table->text('description');
The timeTz method creates a TIME (with timezone) equivalent column with an optional precision (total digits):
$table->timeTz('sunrise', $precision = 0);
The time method creates a TIME equivalent column with an optional precision (total digits):
$table->time('sunrise', $precision = 0);
The timestampTz method creates a TIMESTAMP (with timezone) equivalent column with an optional precision (total digits):
$table->timestampTz('added_at', $precision = 0);
The timestamp method creates a TIMESTAMP equivalent column with an optional precision (total digits):
$table->timestamp('added_at', $precision = 0);
The timestampsTz method creates created_at and updated_at TIMESTAMP (with timezone) equivalent columns with an optional precision (total digits):
$table->timestampsTz($precision = 0);
The timestamps method creates created_at and updated_at TIMESTAMP equivalent columns with an optional precision (total digits):
$table->timestamps($precision = 0);
The tinyIncrements method creates an auto-incrementing UNSIGNED TINYINT equivalent column as a primary key:
$table->tinyIncrements('id');
The tinyInteger method creates a TINYINT equivalent column:
$table->tinyInteger('votes');
The unsignedBigInteger method creates an UNSIGNED BIGINT equivalent column:
$table->unsignedBigInteger('votes');
The unsignedDecimal method creates an UNSIGNED DECIMAL equivalent column with an optional precision (total digits) and scale (decimal digits):
$table->unsignedDecimal('amount', $precision = 8, $scale = 2);
The unsignedInteger method creates an UNSIGNED INTEGER equivalent column:
$table->unsignedInteger('votes');
The unsignedMediumInteger method creates an UNSIGNED MEDIUMINT equivalent column:
$table->unsignedMediumInteger('votes');
The unsignedSmallInteger method creates an UNSIGNED SMALLINT equivalent column:
$table->unsignedSmallInteger('votes');
The unsignedTinyInteger method creates an UNSIGNED TINYINT equivalent column:
$table->unsignedTinyInteger('votes');
The uuidMorphs method is a convenience method that adds a {column}_id CHAR(36) equivalent column and a {column}_type VARCHAR equivalent column.
This method is intended to be used when defining the columns necessary for a polymorphic Eloquent relationship that use UUID identifiers. In the following example, taggable_id and taggable_type columns would be created:
$table->uuidMorphs('taggable');
The uuid method creates a UUID equivalent column:
$table->uuid('id');
The year method creates a YEAR equivalent column:
$table->year('birth_year');
In addition to the column types listed above, there are several column "modifiers" you may use when adding a column to a database table. For example, to make the column "nullable", you may use the nullable method:
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;
Schema::table('users', function (Blueprint $table) {
$table->string('email')->nullable();
});
The following table contains all of the available column modifiers. This list does not include index modifiers:
| Modifier | Description |
|---|---|
->after('column') |
Place the column "after" another column (MySQL). |
->autoIncrement() |
Set INTEGER columns as auto-incrementing (primary key). |
->charset('utf8mb4') |
Specify a character set for the column (MySQL). |
->collation('utf8mb4_unicode_ci') |
Specify a collation for the column (MySQL/PostgreSQL/SQL Server). |
->comment('my comment') |
Add a comment to a column (MySQL/PostgreSQL). |
->default($value) |
Specify a "default" value for the column. |
->first() |
Place the column "first" in the table (MySQL). |
->from($integer) |
Set the starting value of an auto-incrementing field (MySQL / PostgreSQL). |
->nullable($value = true) |
Allow NULL values to be inserted into the column. |
->storedAs($expression) |
Create a stored generated column (MySQL). |
->unsigned() |
Set INTEGER columns as UNSIGNED (MySQL). |
->useCurrent() |
Set TIMESTAMP columns to use CURRENT_TIMESTAMP as default value. |
->useCurrentOnUpdate() |
Set TIMESTAMP columns to use CURRENT_TIMESTAMP when a record is updated. |
->virtualAs($expression) |
Create a virtual generated column (MySQL). |
->generatedAs($expression) |
Create an identity column with specified sequence options (PostgreSQL). |
->always() |
Defines the precedence of sequence values over input for an identity column (PostgreSQL). |
The default modifier accepts a value or an Illuminate\Database\Query\Expression instance. Using an Expression instance will prevent Laravel from wrapping the value in quotes and allow you to use database specific functions. One situation where this is particularly useful is when you need to assign default values to JSON columns:
<?php
use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Query\Expression;
use Illuminate\Database\Migrations\Migration;
class CreateFlightsTable extends Migration
{
/**
* Run the migrations.
*
* @return void
*/
public function up()
{
Schema::create('flights', function (Blueprint $table) {
$table->id();
$table->json('movies')->default(new Expression('(JSON_ARRAY())'));
$table->timestamps();
});
}
}
{note} Support for default expressions depends on your database driver, database version, and the field type. Please refer to your database's documentation.
Before modifying a column, you must install the doctrine/dbal package using the Composer package manager. The Doctrine DBAL library is used to determine the current state of the column and to create the SQL queries needed to make the requested changes to your column:
composer require doctrine/dbal
If you plan to modify columns created using the timestamp method, you must also add the following configuration to your application's config/database.php configuration file:
use Illuminate\Database\DBAL\TimestampType;
'dbal' => [
'types' => [
'timestamp' => TimestampType::class,
],
],{note} If your application is using Microsoft SQL Server, please ensure that you install
doctrine/dbal:^3.0.
The change method allows you to modify the type and attributes of existing columns. For example, you may wish to increase the size of a string column. To see the change method in action, let's increase the size of the name column from 25 to 50. To accomplish this, we simply define the new state of the column and then call the change method:
Schema::table('users', function (Blueprint $table) {
$table->string('name', 50)->change();
});
We could also modify a column to be nullable:
Schema::table('users', function (Blueprint $table) {
$table->string('name', 50)->nullable()->change();
});
{note} The following column types can be modified:
bigInteger,binary,boolean,date,dateTime,dateTimeTz,decimal,integer,json,longText,mediumText,smallInteger,string,text,time,unsignedBigInteger,unsignedInteger,unsignedSmallInteger, anduuid. To modify atimestampcolumn type a Doctrine type must be registered.
To rename a column, you may use the renameColumn method provided by the schema builder blueprint. Before renaming a column, ensure that you have installed the doctrine/dbal library via the Composer package manager:
Schema::table('users', function (Blueprint $table) {
$table->renameColumn('from', 'to');
});
{note} Renaming an
enumcolumn is not currently supported.
To drop a column, you may use the dropColumn method on the schema builder blueprint. If your application is utilizing an SQLite database, you must install the doctrine/dbal package via the Composer package manager before the dropColumn method may be used:
Schema::table('users', function (Blueprint $table) {
$table->dropColumn('votes');
});
You may drop multiple columns from a table by passing an array of column names to the dropColumn method:
Schema::table('users', function (Blueprint $table) {
$table->dropColumn(['votes', 'avatar', 'location']);
});
{note} Dropping or modifying multiple columns within a single migration while using an SQLite database is not supported.
Laravel provides several convenient methods related to dropping common types of columns. Each of these methods is described in the table below:
| Command | Description |
|---|---|
$table->dropMorphs('morphable'); |
Drop the morphable_id and morphable_type columns. |
$table->dropRememberToken(); |
Drop the remember_token column. |
$table->dropSoftDeletes(); |
Drop the deleted_at column. |
$table->dropSoftDeletesTz(); |
Alias of dropSoftDeletes() method. |
$table->dropTimestamps(); |
Drop the created_at and updated_at columns. |
$table->dropTimestampsTz(); |
Alias of dropTimestamps() method. |
The Laravel schema builder supports several types of indexes. The following example creates a new email column and specifies that its values should be unique. To create the index, we can chain the unique method onto the column definition:
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;
Schema::table('users', function (Blueprint $table) {
$table->string('email')->unique();
});
Alternatively, you may create the index after defining the column. To do so, you should call the unique method on the schema builder blueprint. This method accepts the name of the column that should receive a unique index:
$table->unique('email');
You may even pass an array of columns to an index method to create a compound (or composite) index:
$table->index(['account_id', 'created_at']);
When creating an index, Laravel will automatically generate an index name based on the table, column names, and the index type, but you may pass a second argument to the method to specify the index name yourself:
$table->unique('email', 'unique_email');
Laravel's schema builder blueprint class provides methods for creating each type of index supported by Laravel. Each index method accepts an optional second argument to specify the name of the index. If omitted, the name will be derived from the names of the table and column(s) used for the index, as well as the index type. Each of the available index methods is described in the table below:
| Command | Description |
|---|---|
$table->primary('id'); |
Adds a primary key. |
$table->primary(['id', 'parent_id']); |
Adds composite keys. |
$table->unique('email'); |
Adds a unique index. |
$table->index('state'); |
Adds an index. |
$table->spatialIndex('location'); |
Adds a spatial index (except SQLite). |
By default, Laravel uses the utf8mb4 character set. If you are running a version of MySQL older than the 5.7.7 release or MariaDB older than the 10.2.2 release, you may need to manually configure the default string length generated by migrations in order for MySQL to create indexes for them. You may configure the default string length by calling the Schema::defaultStringLength method within the boot method of your App\Providers\AppServiceProvider class:
use Illuminate\Support\Facades\Schema;
/**
* Bootstrap any application services.
*
* @return void
*/
public function boot()
{
Schema::defaultStringLength(191);
}
Alternatively, you may enable the innodb_large_prefix option for your database. Refer to your database's documentation for instructions on how to properly enable this option.
To rename an index, you may use the renameIndex method provided by the schema builder blueprint. This method accepts the current index name as its first argument and the desired name as its second argument:
$table->renameIndex('from', 'to')
To drop an index, you must specify the index's name. By default, Laravel automatically assigns an index name based on the table name, the name of the indexed column, and the index type. Here are some examples:
| Command | Description |
|---|---|
$table->dropPrimary('users_id_primary'); |
Drop a primary key from the "users" table. |
$table->dropUnique('users_email_unique'); |
Drop a unique index from the "users" table. |
$table->dropIndex('geo_state_index'); |
Drop a basic index from the "geo" table. |
$table->dropSpatialIndex('geo_location_spatialindex'); |
Drop a spatial index from the "geo" table (except SQLite). |
If you pass an array of columns into a method that drops indexes, the conventional index name will be generated based on the table name, columns, and index type:
Schema::table('geo', function (Blueprint $table) {
$table->dropIndex(['state']); // Drops index 'geo_state_index'
});
Laravel also provides support for creating foreign key constraints, which are used to force referential integrity at the database level. For example, let's define a user_id column on the posts table that references the id column on a users table:
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;
Schema::table('posts', function (Blueprint $table) {
$table->unsignedBigInteger('user_id');
$table->foreign('user_id')->references('id')->on('users');
});
Since this syntax is rather verbose, Laravel provides additional, terser methods that use conventions to provide a better developer experience. The example above can be rewritten like so:
Schema::table('posts', function (Blueprint $table) {
$table->foreignId('user_id')->constrained();
});
The foreignId method is an alias for unsignedBigInteger while the constrained method will use conventions to determine the table and column name being referenced. If your table name does not match Laravel's conventions, you may specify the table name by passing it as an argument to the constrained method:
Schema::table('posts', function (Blueprint $table) {
$table->foreignId('user_id')->constrained('users');
});
You may also specify the desired action for the "on delete" and "on update" properties of the constraint:
$table->foreignId('user_id')
->constrained()
->onUpdate('cascade')
->onDelete('cascade');
Any additional column modifiers must be called before the constrained method:
$table->foreignId('user_id')
->nullable()
->constrained();
To drop a foreign key, you may use the dropForeign method, passing the name of the foreign key constraint to be deleted as an argument. Foreign key constraints use the same naming convention as indexes. In other words, the foreign key constraint name is based on the name of the table and the columns in the constraint, followed by a "_foreign" suffix:
$table->dropForeign('posts_user_id_foreign');
Alternatively, you may pass an array containing the column name that holds the foreign key to the dropForeign method. The array will be converted to a foreign key constraint name using Laravel's constraint naming conventions:
$table->dropForeign(['user_id']);
You may enable or disable foreign key constraints within your migrations by using the following methods:
Schema::enableForeignKeyConstraints();
Schema::disableForeignKeyConstraints();
{note} SQLite disables foreign key constraints by default. When using SQLite, make sure to enable foreign key support in your database configuration before attempting to create them in your migrations. In addition, SQLite only supports foreign keys upon creation of the table and not when tables are altered.