Introduction to SQL Questions and Answers

Q: ORDER BY can be combined with the SELECT statements.

Correct answer: TrueORDER BY is a clause of the SELECT statement and comes last, after WHERE, GROUP BY and HAVING - sorting is the final operation applied to what those have produced.It is worth combining deliberately rather than by habit: without it the result has no defined order at all, because a relation is a set. Any order observed without it is incidental to how the engine happened to read the data, and it can change when the table grows or an index is added.

Q: Data manipulation language (DML) commands are used to define a database, including creating, altering, and dropping tables and establishing constraints.

Correct answer: FalseThe sentence describes DDL, the Data Definition Language, not DML. Creating, altering and dropping tables and establishing constraints are all structural operations.DML works with the data inside those structures: SELECT, INSERT, UPDATE and DELETE. The distinction has a practical edge beyond terminology - in most engines DDL statements commit implicitly and cannot be rolled back, while DML statements participate in a transaction.

Q: Which of the following is the original purpose of SQL?

Correct answer: All of the above.SQL was designed as a complete language for relational databases, so it covers all three.The data definition half defines structures - CREATE, ALTER, DROP. The manipulation half queries and changes data - SELECT, INSERT, UPDATE, DELETE. Defining the data structures is what the first half achieves. That combination is what made SQL displace the separate languages the earlier database models required, and it is why one standard covers both syntax and semantics.

Q: Delete from r; r - relation This command performs which of the following action ?

Correct answer: Clear relation entriesDELETE FROM r with no WHERE clause removes every tuple in the relation, leaving the relation itself in place - defined, empty, and ready for new rows.That is the precise sense in which the entries are cleared rather than the relation removed: DROP TABLE r would be needed for the latter. Nor are fields affected, since DELETE works on whole rows and never on the column structure. TRUNCATE TABLE achieves the same emptying more quickly, without logging each row.

Q: SQL provides five built-in functions: COUNT, SUM, AVG, MAX, MIN.

Correct answer: TrueCOUNT, SUM, AVG, MAX and MIN are the five aggregate functions defined by the standard, each reducing a set of rows to a single value.They are typically used with GROUP BY, which computes one value per group, and filtered with HAVING. All five ignore nulls, with the exception of COUNT(*), which counts rows whatever they contain. Individual products add many more - standard deviation, string aggregation, statistical functions - but those five are the standard set.

Q: The ADD command is used to enter one row of data or to add multiple rows as a result of a query.

Correct answer: FalseThere is no ADD statement in SQL. The statement that enters rows is INSERT - a single row with VALUES, or many at once with a SELECT supplying them, which is exactly the behaviour the sentence describes.ADD appears only as a keyword within ALTER TABLE ... ADD COLUMN, which changes the table's structure rather than its contents. The sentence is a correct description of INSERT attached to the wrong keyword.

Q: Which one of the following is used to define the structure of the relation ,deleting relations and relating schemas ?

Correct answer: DDL(Data Definition Langauge)Defining a relation's structure, deleting relations and relating schemas are all structural operations, and they belong to the Data Definition Language - CREATE, ALTER and DROP.DML works on the data those structures hold. A query is one DML statement rather than a language, and a relational schema is the description DDL produces, not the means of producing it. A practical consequence of the split: in most engines DDL commits implicitly and cannot be rolled back.

Q: COUNT(field_name) tallies only those rows that contain a value; it ignores all null values.

Correct answer: TrueCOUNT(field_name) counts rows in which that field is not null, so nulls reduce the count.COUNT(*) behaves differently, counting rows regardless of their contents, and the gap between the two is a quick way to find how many nulls a column holds. The same null-skipping applies to SUM, AVG, MIN and MAX, which matters most for AVG: it divides by the number of non-null values, not by the number of rows.

51.

ORDER BY can be combined with the SELECT statements.

A.True
B.False

Answer: A

Correct answer: True

ORDER BY is a clause of the SELECT statement and comes last, after WHERE, GROUP BY and HAVING - sorting is the final operation applied to what those have produced.

It is worth combining deliberately rather than by habit: without it the result has no defined order at all, because a relation is a set. Any order observed without it is incidental to how the engine happened to read the data, and it can change when the table grows or an index is added.

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52.

Data manipulation language (DML) commands are used to define a database, including creating, altering, and dropping tables and establishing constraints.

A.True
B.False

Answer: B

Correct answer: False

The sentence describes DDL, the Data Definition Language, not DML. Creating, altering and dropping tables and establishing constraints are all structural operations.

DML works with the data inside those structures: SELECT, INSERT, UPDATE and DELETE. The distinction has a practical edge beyond terminology - in most engines DDL statements commit implicitly and cannot be rolled back, while DML statements participate in a transaction.

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53.

The wildcard in a WHERE clause is useful when?

A.An exact match is necessary in a SELECT statement.
B.An exact match is not possible in a SELECT statement.
C.An exact match is necessary in a CREATE statement.
D.An exact match is not possible in a CREATE statement.

Answer: B

Correct answer: An exact match is not possible in a SELECT statement.

Wildcards exist for the case where the exact value is unknown or where a pattern rather than a value is wanted - every surname beginning with "Mc", every product code with a particular third character.

Where the value is known exactly, = is both simpler and faster, since an equality test can use an index while a pattern beginning with a wildcard generally cannot. Wildcards belong to LIKE in a WHERE clause, so they have no place in a CREATE statement, which defines structure rather than selecting data.

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54.

Which of the following is the original purpose of SQL?

A.To specify the syntax and semantics of SQL data definition language
B.To specify the syntax and semantics of SQL manipulation language
C.To define the data structures
D.All of the above.

Answer: D

Correct answer: All of the above.

SQL was designed as a complete language for relational databases, so it covers all three.

The data definition half defines structures - CREATE, ALTER, DROP. The manipulation half queries and changes data - SELECT, INSERT, UPDATE, DELETE. Defining the data structures is what the first half achieves. That combination is what made SQL displace the separate languages the earlier database models required, and it is why one standard covers both syntax and semantics.

Discuss in Forum

55.

Delete from r; r - relation This command performs which of the following action ?

A.Remove relation
B.Clear relation entries
C.Delete fields
D.Delete rows

Answer: B

Correct answer: Clear relation entries

DELETE FROM r with no WHERE clause removes every tuple in the relation, leaving the relation itself in place - defined, empty, and ready for new rows.

That is the precise sense in which the entries are cleared rather than the relation removed: DROP TABLE r would be needed for the latter. Nor are fields affected, since DELETE works on whole rows and never on the column structure. TRUNCATE TABLE achieves the same emptying more quickly, without logging each row.

Discuss in Forum

56.

SQL provides five built-in functions: COUNT, SUM, AVG, MAX, MIN.

A.True
B.False

Answer: A

Correct answer: True

COUNT, SUM, AVG, MAX and MIN are the five aggregate functions defined by the standard, each reducing a set of rows to a single value.

They are typically used with GROUP BY, which computes one value per group, and filtered with HAVING. All five ignore nulls, with the exception of COUNT(*), which counts rows whatever they contain. Individual products add many more - standard deviation, string aggregation, statistical functions - but those five are the standard set.

Discuss in Forum

57.

The ADD command is used to enter one row of data or to add multiple rows as a result of a query.

A.True
B.False

Answer: B

Correct answer: False

There is no ADD statement in SQL. The statement that enters rows is INSERT - a single row with VALUES, or many at once with a SELECT supplying them, which is exactly the behaviour the sentence describes.

ADD appears only as a keyword within ALTER TABLE ... ADD COLUMN, which changes the table's structure rather than its contents. The sentence is a correct description of INSERT attached to the wrong keyword.

Discuss in Forum

58.

A view is which of the following?

A.A virtual table that can be accessed via SQL commands
B.A virtual table that cannot be accessed via SQL commands
C.A base table that can be accessed via SQL commands
D.A base table that cannot be accessed via SQL commands

Answer: A

Correct answer: A virtual table that can be accessed via SQL commands

A view is a stored query that behaves like a table. It has a name, columns and rows, and can be queried with SELECT exactly as a table can - but it stores no data of its own, deriving its contents from the underlying tables each time it is referenced.

That is what "virtual" means here, and it is the difference from a base table, which does hold data. Views serve two purposes in practice: they hide complexity, presenting a join as a single object, and they restrict access, exposing only chosen rows and columns.

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59.

Which one of the following is used to define the structure of the relation ,deleting relations and relating schemas ?

A.DML(Data Manipulation Langauge)
B.DDL(Data Definition Langauge)
C.Query
D.Relational Schema

Answer: B

Correct answer: DDL(Data Definition Langauge)

Defining a relation's structure, deleting relations and relating schemas are all structural operations, and they belong to the Data Definition Language - CREATE, ALTER and DROP.

DML works on the data those structures hold. A query is one DML statement rather than a language, and a relational schema is the description DDL produces, not the means of producing it. A practical consequence of the split: in most engines DDL commits implicitly and cannot be rolled back.

Discuss in Forum

60.

COUNT(field_name) tallies only those rows that contain a value; it ignores all null values.

A.True
B.False

Answer: A

Correct answer: True

COUNT(field_name) counts rows in which that field is not null, so nulls reduce the count.

COUNT(*) behaves differently, counting rows regardless of their contents, and the gap between the two is a quick way to find how many nulls a column holds. The same null-skipping applies to SUM, AVG, MIN and MAX, which matters most for AVG: it divides by the number of non-null values, not by the number of rows.

Discuss in Forum

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