Which of the following SQL statements correctly adds a NOT NULL constraint to an existing column phone in table contacts?

easy📝 Syntax Q3 of 15

SQL - Table Constraints

Which of the following SQL statements correctly adds a NOT NULL constraint to an existing column phone in table contacts?

AALTER TABLE contacts ADD NOT NULL phone;

BALTER TABLE contacts CHANGE phone NOT NULL;

CALTER TABLE contacts SET phone NOT NULL;

DALTER TABLE contacts MODIFY phone VARCHAR(15) NOT NULL;

Step-by-Step Solution

Solution:

Step 1: Understand how to alter column constraints
To add NOT NULL, you modify the column definition with the data type and NOT NULL.
Step 2: Check syntax correctness
ALTER TABLE contacts MODIFY phone VARCHAR(15) NOT NULL; uses correct syntax: ALTER TABLE ... MODIFY column datatype NOT NULL. Others are invalid or incomplete.
Final Answer:
ALTER TABLE contacts MODIFY phone VARCHAR(15) NOT NULL; -> Option D
Quick Check:
Use MODIFY with datatype and NOT NULL to alter column [OK]

Quick Trick: Use MODIFY with datatype to add NOT NULL [OK]

Common Mistakes:

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