In SQL Data Definition Language (DDL), what can the ALTER TABLE statement be used to do?

Introduction / Context:
SQL separates data definition (DDL) from data manipulation (DML). ALTER TABLE is a DDL command used to evolve schema safely as requirements change.

Given Data / Assumptions:

• We want to modify a table's structure or constraints.
• We are not directly editing row contents via ALTER.
• Standard SQL semantics apply.

Concept / Approach:
ALTER TABLE alters schema objects: add/drop columns, change data types where allowed, add/drop constraints (PRIMARY KEY, UNIQUE, FOREIGN KEY, CHECK), rename objects (vendor-specific), or adjust storage options. DML like UPDATE, INSERT, DELETE manipulates data, not schema.

Step-by-Step Solution:

Verification / Alternative check:
RDBMS documentation classifies ALTER TABLE under DDL, distinct from DML statements.

Why Other Options Are Wrong:
Change table data / add rows / delete rows: Performed by UPDATE, INSERT, DELETE respectively.
Export data: Handled by utilities (COPY, bcp) or SELECT INTO OUTFILE (vendor-specific), not ALTER.

Common Pitfalls:
Altering types or constraints on large tables can lock or rewrite data; plan maintenance windows and backups.

Final Answer:
Change the table structure (add/modify/drop columns, constraints, etc.)