Introduction to SQL Questions and Answers

Q: The basic data type char(n) is a _____ length character string and varchar(n) is _____ length character.

Correct answer: Fixed, variablechar(n) always occupies \( n \) characters: a shorter value is padded with trailing spaces to fill the width. varchar(n) stores only the characters actually supplied, up to a maximum of \( n \).The trade is space against predictability. char wastes space on short values but has a fixed offset, which can make row access marginally faster; varchar stores just what is needed at the cost of a length prefix. The practical rule is char for genuinely fixed-width codes and varchar for anything that varies.

Q: The SQL keyword BETWEEN is used:

Correct answer: for ranges.BETWEEN tests whether a value falls within a range, and the range is inclusive at both ends: WHERE age BETWEEN 18 AND 65 is the same as age >= 18 AND age <= 65.That inclusivity is the point most often got wrong, particularly with dates, where BETWEEN '2026-01-01' AND '2026-01-31' excludes anything timestamped later on the last day. It filters rows, so it does not limit columns, and it takes exact values rather than patterns, so it is not a wildcard construct.

Q: The SELECT command, with its various clauses, allows users to query the data contained in the tables and ask many different questions or ad hoc queries.

Correct answer: TrueThis is what distinguishes a relational database from the file-based systems it replaced. SELECT, with its clauses for filtering, grouping, ordering and joining, lets a user pose a question the database designer never anticipated.That is what "ad hoc" means here: the query is composed when the question arises, not built into the application in advance. Earlier models required navigating predefined access paths, so a new question often meant new code. Declaring what is wanted rather than how to fetch it is the whole point.

Q: The command to remove rows from a table 'CUSTOMER' is:

Correct answer: DELETE FROM CUSTOMER WHERE .DELETE FROM removes rows, and the WHERE clause decides which. Omitting the clause is legal and empties the whole table, which is why writing the WHERE first is a habit worth having.DROP destroys the table itself, structure and all, so it belongs to the data definition language rather than to row manipulation. UPDATE changes values in rows that remain. REMOVE is not SQL. Note that DELETE leaves the empty table in place, ready for new rows.

Q: The HAVING clause acts like a WHERE clause, but it identifies groups that meet a criterion, rather than rows.

Correct answer: TrueHAVING is the group-level counterpart of WHERE. It runs after GROUP BY has formed the groups and after the aggregates have been computed, so it can test them: HAVING COUNT(*) > 5 keeps only groups with more than five rows.WHERE runs earlier, deciding which rows enter the groups, and cannot refer to an aggregate because at that point no group exists. Filtering on a plain column therefore belongs in WHERE, where it reduces the rows to be grouped, not in HAVING.

Q: The SQL statement that queries or reads data from a table is ________ .

Correct answer: SELECTSELECT is the query statement, and the only one that reads data. It is also by far the most elaborate, carrying FROM, WHERE, GROUP BY, HAVING and ORDER BY, and able to nest inside itself.READ and QUERY are not SQL keywords. The choice of word is a historical accident worth noting: SELECT suggests choosing rows, but in relational terms the statement performs projection as well as selection, and after GROUP BY it aggregates too.

Q: The command to eliminate a table from a database is:

Correct answer: DROP TABLE CUSTOMER;DROP TABLE removes the table itself - its rows, its definition, its indexes and its constraints - leaving no object of that name in the database.The contrast with DELETE is the point of the question. DELETE FROM CUSTOMER removes rows but keeps the table, ready for new ones. DROP is data definition, DELETE is data manipulation. TRUNCATE TABLE sits between them, emptying the table quickly while keeping its structure. REMOVE TABLE and DELETE TABLE are not SQL.

Q: Which one of the following provides the ability to query information from the database and to insert tuples into, delete tuples from, and modify tuples in the database ?

Correct answer: DML(Data Manipulation Langauge)Querying, inserting, deleting and modifying tuples are exactly the operations of the Data Manipulation Language: SELECT, INSERT, DELETE and UPDATE.DDL by contrast defines the structures those tuples live in. Note that the query language is a part of DML rather than a separate thing, which is why "Query" as an option is too narrow - it names one of the four capabilities the question lists. A relational schema is a description, not a language.

61.

To define what columns should be displayed in an SQL SELECT statement:

A.use FROM to name the source table(s) and list the columns to be shown after SELECT.
B.use USING to name the source table(s) and list the columns to be shown after SELECT.
C.use SELECT to name the source table(s) and list the columns to be shown after USING.
D.use USING to name the source table(s) and list the columns to be shown after WHERE.

Answer: A

Correct answer: use FROM to name the source table(s) and list the columns to be shown after SELECT.

Each clause has one job. FROM names the source - a table, several joined tables, a view or a subquery - and the SELECT list names the columns to be returned from it.

In relational terms these are the two basic operations: SELECT performs projection, choosing columns, and WHERE performs selection, choosing rows. USING exists in SQL but for a narrower purpose - naming the shared column in a join, as in JOIN orders USING (customer_id) - and never names the source table.

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62.

The basic data type char(n) is a _____ length character string and varchar(n) is _____ length character.

A.Fixed, equal
B.Equal, variable
C.Fixed, variable
D.Variable, equal

Answer: C

Correct answer: Fixed, variable

char(n) always occupies \( n \) characters: a shorter value is padded with trailing spaces to fill the width. varchar(n) stores only the characters actually supplied, up to a maximum of \( n \).

The trade is space against predictability. char wastes space on short values but has a fixed offset, which can make row access marginally faster; varchar stores just what is needed at the cost of a length prefix. The practical rule is char for genuinely fixed-width codes and varchar for anything that varies.

Discuss in Forum

63.

The SQL keyword BETWEEN is used:

A.for ranges.
B.to limit the columns displayed.
C.as a wildcard.
D.None of the above is correct.

Answer: A

Correct answer: for ranges.

BETWEEN tests whether a value falls within a range, and the range is inclusive at both ends: WHERE age BETWEEN 18 AND 65 is the same as age >= 18 AND age <= 65.

That inclusivity is the point most often got wrong, particularly with dates, where BETWEEN '2026-01-01' AND '2026-01-31' excludes anything timestamped later on the last day. It filters rows, so it does not limit columns, and it takes exact values rather than patterns, so it is not a wildcard construct.

Discuss in Forum

64.

The SELECT command, with its various clauses, allows users to query the data contained in the tables and ask many different questions or ad hoc queries.

A.True
B.False

Answer: A

Correct answer: True

This is what distinguishes a relational database from the file-based systems it replaced. SELECT, with its clauses for filtering, grouping, ordering and joining, lets a user pose a question the database designer never anticipated.

That is what "ad hoc" means here: the query is composed when the question arises, not built into the application in advance. Earlier models required navigating predefined access paths, so a new question often meant new code. Declaring what is wanted rather than how to fetch it is the whole point.

Discuss in Forum

65.

The command to remove rows from a table 'CUSTOMER' is:

A.REMOVE FROM CUSTOMER .
B.DROP FROM CUSTOMER .
C.DELETE FROM CUSTOMER WHERE .
D.UPDATE FROM CUSTOMER .

Answer: C

Correct answer: DELETE FROM CUSTOMER WHERE .

DELETE FROM removes rows, and the WHERE clause decides which. Omitting the clause is legal and empties the whole table, which is why writing the WHERE first is a habit worth having.

DROP destroys the table itself, structure and all, so it belongs to the data definition language rather than to row manipulation. UPDATE changes values in rows that remain. REMOVE is not SQL. Note that DELETE leaves the empty table in place, ready for new rows.

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66.

The HAVING clause acts like a WHERE clause, but it identifies groups that meet a criterion, rather than rows.

A.True
B.False

Answer: A

Correct answer: True

HAVING is the group-level counterpart of WHERE. It runs after GROUP BY has formed the groups and after the aggregates have been computed, so it can test them: HAVING COUNT(*) > 5 keeps only groups with more than five rows.

WHERE runs earlier, deciding which rows enter the groups, and cannot refer to an aggregate because at that point no group exists. Filtering on a plain column therefore belongs in WHERE, where it reduces the rows to be grouped, not in HAVING.

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67.

The SQL statement that queries or reads data from a table is ________ .

A.SELECT
B.READ
C.QUERY
D.None of the above is correct.

Answer: A

Correct answer: SELECT

SELECT is the query statement, and the only one that reads data. It is also by far the most elaborate, carrying FROM, WHERE, GROUP BY, HAVING and ORDER BY, and able to nest inside itself.

READ and QUERY are not SQL keywords. The choice of word is a historical accident worth noting: SELECT suggests choosing rows, but in relational terms the statement performs projection as well as selection, and after GROUP BY it aggregates too.

Discuss in Forum

68.

Find the SQL statement below that is equal to the following: SELECT NAME FROM CUSTOMER WHERE STATE = 'VA';

A.SELECT NAME IN CUSTOMER WHERE STATE IN ('VA');
B.SELECT NAME IN CUSTOMER WHERE STATE = 'VA';
C.SELECT NAME IN CUSTOMER WHERE STATE = 'V';
D.SELECT NAME FROM CUSTOMER WHERE STATE IN ('VA');

Answer: D

Correct answer: SELECT NAME FROM CUSTOMER WHERE STATE IN ('VA');

IN tests membership of a list, and a list with one element is equivalent to an equality test, so STATE IN ('VA') means the same as STATE = 'VA'.

The other three options fail on syntax rather than on logic: they write IN CUSTOMER where the source table must follow FROM. That is the real test here - the clause naming the table is FROM, and IN is a predicate belonging inside WHERE.

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69.

The command to eliminate a table from a database is:

A.REMOVE TABLE CUSTOMER;
B.DROP TABLE CUSTOMER;
C.DELETE TABLE CUSTOMER;
D.UPDATE TABLE CUSTOMER;

Answer: B

Correct answer: DROP TABLE CUSTOMER;

DROP TABLE removes the table itself - its rows, its definition, its indexes and its constraints - leaving no object of that name in the database.

The contrast with DELETE is the point of the question. DELETE FROM CUSTOMER removes rows but keeps the table, ready for new ones. DROP is data definition, DELETE is data manipulation. TRUNCATE TABLE sits between them, emptying the table quickly while keeping its structure. REMOVE TABLE and DELETE TABLE are not SQL.

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70.

Which one of the following provides the ability to query information from the database and to insert tuples into, delete tuples from, and modify tuples in the database ?

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

Answer: A

Correct answer: DML(Data Manipulation Langauge)

Querying, inserting, deleting and modifying tuples are exactly the operations of the Data Manipulation Language: SELECT, INSERT, DELETE and UPDATE.

DDL by contrast defines the structures those tuples live in. Note that the query language is a part of DML rather than a separate thing, which is why "Query" as an option is too narrow - it names one of the four capabilities the question lists. A relational schema is a description, not a language.

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