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# MySQL

## Usage

### Alter table

#### Add a column to a table

We can add a column to a table with a specified order with the keyword `FIRST` to the front of all columns or `` AFTER `exist_column_name` `` to after the column `` `exist_column_name` ``.

```sql
ALTER TABLE `table_name` ADD COLUMN `new_column_name` VARCHAR ( 255 ) NOT NULL DEFAULT '' COMMENT 'some_comments' AFTER `exist_column_name`;
```

#### Change the data type of a column

```sql
ALTER TABLE `table_name` MODIFY COLUMN `exist_column_name` VARCHAR ( 255 ) NOT NULL DEFAULT '' COMMENT 'some_comments';
```