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ALTER TABLE table_name ALTER COLUMN column_name DATA_TYPE [(COLUMN_SIZE)] NULL; In this syntax: First, specify the name of the table from which you want to change the column. Second, specify the column name with size which you want to change to allow NULL and then write NULL statement .
All you need to do is to replace [Table] with the name of your table, [Col] with the name of your column and TYPE with the datatype of the column. Execute the command and you are allowed to use NULL values for the specified column. That is all it takes to switch between NULL and NOT NULL .
Go to the Object Explorer, Tables View, Right click on the table you want to change, Select Alter Table option, Select Columns option in the left panel, Then check the not null checkboxes in the right, then click ok.
To enforce NOT NULL for a column in MySQL, you use the ALTER TABLE .... MODIFY command and restate the column definition, adding the NOT NULL attribute.
This should be correct:
ALTER TABLE mytable
ALTER COLUMN col TYPE character varying(15),
ALTER COLUMN col SET NOT NULL
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