MySQL add a NOT NULL column

Question

I am adding a column to a table:

ALTER TABLE t ADD c varchar(10) NOT NULL;

The column gets added, and each record has the empty string.

Answer 1

In MySQL, each column type has an "implicit default" value.

For string types [the implicit] default value is the empty string.

If a NOT NULL column is added to a table, and no explicit DEFAULT is specified, the implicit default value is used to populate the new column data¹. Similar rules apply when the DEFAULT value is specified.

As such, the original DDL produces the same results as:

-- After this, data will be the same, but schema has an EXPLICIT DEFAULT
ALTER TABLE t ADD c varchar(10) NOT NULL DEFAULT ''
-- Now we're back to the IMPLICIT DEFAULT (MySQL stores NULL internally)
ALTER TABLE t ALTER c DROP DEFAULT

The "strict" mode settings affects DML statements relying on default values, but do not affect the implicit default usage when the column is added.

For data entry into a NOT NULL column that has no explicit DEFAULT clause, if an INSERT or REPLACE statement includes no value for the column [and if] strict SQL mode is enabled, an error occurs ..

Here is an sqlfiddle "proof" that strict mode does not apply to the ALTER TABLE .. ADD statement.

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¹ This is a MySQL feature. Other engines, like SQL Server, require an explicit DEFAULT (or NULL column) constraint for such schema changes.