Column aliases rename output columns without touching the table — AS keyword syntax and when it is optional

Aliases are available in ORDER BY only — WHERE and HAVING cannot see SELECT aliases (execution order rule)

Arithmetic expressions: +, -, *, /, % with correct operator precedence and parentheses

String concatenation cross-database: || (PostgreSQL/Oracle), CONCAT() (MySQL), + (SQL Server — not NULL-safe)

NULL propagation: any NULL operand silently nullifies the entire expression — use COALESCE as a shield

Non-SARGable expressions: functions on indexed columns in WHERE cause full scans — a top production bug

Table aliases shorten JOIN queries and prevent ambiguous column references across multiple tables

Production ETL patterns: ROUND, discount formulas, commission calculations with proper precedence

Column aliases rename output columns without touching the table — AS keyword syntax and when it is optional

Aliases are available in ORDER BY only — WHERE and HAVING cannot see SELECT aliases (execution order rule)

Arithmetic expressions: +, -, *, /, % with correct operator precedence and parentheses

String concatenation cross-database: || (PostgreSQL/Oracle), CONCAT() (MySQL), + (SQL Server — not NULL-safe)

NULL propagation: any NULL operand silently nullifies the entire expression — use COALESCE as a shield

Non-SARGable expressions: functions on indexed columns in WHERE cause full scans — a top production bug

Table aliases shorten JOIN queries and prevent ambiguous column references across multiple tables

Production ETL patterns: ROUND, discount formulas, commission calculations with proper precedence