SQL Fundamentals & Data Manipulation Flashcards

Which clause in a SELECT statement is used to combine rows from two or more tables based on a related column?

Answer: JOIN ... ON

JOIN ... ON links two tables by matching rows where the ON condition is true, producing a combined result set.

What does ROLLBACK do in Oracle SQL?

Answer: Undoes all uncommitted DML changes since the last COMMIT or SAVEPOINT

ROLLBACK reverses all uncommitted DML operations back to the last COMMIT or to a named SAVEPOINT.

Which SQL operator returns rows that exist in the first SELECT result but NOT in the second?

Answer: MINUS

Oracle's MINUS operator returns distinct rows from the first query that do not appear in the second query's result.

Which of the following correctly adds a new column named phone_num to an existing table?

Answer: ALTER TABLE employees ADD phone_num VARCHAR2(20);

ALTER TABLE ... ADD is the DDL syntax used to add a new column to an existing table.

What value does AVG(salary) return if a group contains salaries 1000, 2000, NULL, and 3000?

Answer: 2000

AVG ignores NULL values, so it divides the sum (6000) by the count of non-NULL values (3), returning 2000.

Which SELECT clause is evaluated FIRST by the Oracle SQL engine during query processing?

Answer: FROM

Oracle logically processes FROM first to identify the data source(s), then WHERE, then SELECT, then ORDER BY.

Which statement about the CHAR and VARCHAR2 data types in Oracle is correct?

Answer: CHAR stores fixed-length strings padded with spaces; VARCHAR2 stores variable-length strings

CHAR is fixed-length and pads with trailing spaces, while VARCHAR2 stores only the actual characters entered.