The ALTER TABLE Statement
To change the data type, or the size of a table column we have to use the ALTER TABLE statement.
The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.
The ALTER TABLE statement is also used to add and drop various constraints on an existing table.
ALTER COLUMN
We want to change the data type of the year column of the cars table from INT to VARCHAR(4).
To modify a column, use the ALTER COLUMN statement and the TYPE keyword followed by the new data type:
Example
ALTER TABLE cars
ALTER COLUMN
year TYPE VARCHAR(4);
Result
ALTER TABLE
Note: Some data types cannot be converted if the column has value. E.g. numbers can always be converted to text, but text cannot always be converted to numbers.
Change Maximum Allowed Characters
We also want to change the maximum number of characters allowed in the color column of the cars table.
Use the same syntax as above, use the ALTER COLUMN statement and the TYPE keyword followed by the new data type:
Example
ALTER TABLE cars
ALTER COLUMN color TYPE VARCHAR(30);
Result
ALTER TABLE
