Top 20 SQL interview questions & Answers for Testers

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Here are the top 20 SQL interview questions and
answers for testers 1) What is SQL and its
significance in software testing?  SQL
(Structured Query Language) is a programming
language used to manage and manipulate relational
databases. Testers often use SQL to query
databases, validate data, and perform
data-related tests. 2) What are the different
types of SQL statements?  SQL statements can be
classified into four main types Data
Manipulation Language (DML) Used to manipulate
data in the database (e.g., SELECT, INSERT,
UPDATE, DELETE). Data Definition Language (DDL)
Used to define and modify database structures
(e.g., CREATE, ALTER, DROP). Data Control
Language (DCL) Used to control access and
permissions on the database (e.g., GRANT,
REVOKE). Transaction Control Language (TCL) Used
to manage transactions in the database (e.g.,
COMMIT, ROLLBACK).
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3) What is a primary key?  A primary key is a
unique identifier for a row in a table. It
ensures that each row can be uniquely identified
and helps maintain data integrity. 4) What is a
foreign key?  A foreign key is a field in a table
that refers to the primary key of another table.
It establishes a relationship between two tables,
ensuring referential integrity. 5) What is
normalization?  Normalization is the process of
organizing data in a database to reduce
redundancy and improve data integrity. It
involves breaking down tables into smaller, more
manageable structures.
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6) What is a stored procedure?  A stored
procedure is a set of SQL statements that are
stored and executed on the database server. It
can be called and executed multiple times,
reducing network traffic and improving
performance. 7) What is a view in SQL?  A view
is a virtual table derived from one or more
tables. It does not store data but presents the
data in a predefined manner, simplifying complex
queries. 8) What is the difference between
UNION and UNION ALL?  UNION combines the result
sets of two or more SELECT statements, removing
duplicate rows. UNION ALL also combines result
sets but includes all rows, including
duplicates.
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9) What is an index?  An index is a database
structure that improves the speed of data
retrieval operations. It allows faster searching,
sorting, and filtering of data. 10) What is the
difference between a clustered and a
non-clustered index?  A clustered index
determines the physical order of data rows in a
table, while a non-clustered index is a separate
structure that stores a copy of the indexed
columns along with a pointer to the actual
row. 11) What is the purpose of the GROUP BY
clause?  The GROUP BY clause is used to group
rows based on one or more columns. It is often
used with aggregate functions like COUNT, SUM,
AVG, etc., to perform calculations on groups of
data.
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12) What is a self-join?  A self-join is a join
operation where a table is joined with itself. It
is useful when there is a need to compare rows
within the same table. 13) Explain the
difference between the WHERE and HAVING
clauses.  The WHERE clause is used to filter rows
before grouping, while the HAVING clause is used
to filter groups after grouping. 14) What is a
subquery?  A subquery is a query nested within
another query. It is used to retrieve data based
on the results of another query. 15) What is the
purpose of the EXISTS operator?  The EXISTS
operator is used to check the existence of rows
returned by a subquery. It returns true if the
subquery returns any rows otherwise, it returns
false.
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16) What is the ACID property in database
systems?  ACID stands for Atomicity, Consistency,
Isolation, and Durability. It defines the
properties that ensure reliable processing of
database transactions. 17) Explain the
difference between a full backup and an
incremental backup.  A full backup copies all
data in a database, while an incremental backup
only copies the changes made since the last
backup. Incremental backups are faster but
require more effort to restore 18) What is a
deadlock?  A deadlock is a situation where two or
more transactions are waiting for each other to
release resources, resulting in a permanent state
of inactivity. It can lead to a system freeze or
deadlock resolution by a database management
system.
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19) How can you optimize a SQL query?  SQL query
optimization can be achieved through various
techniques like creating indexes, minimizing the
use of subqueries, using efficient join methods,
and retrieving only the necessary columns. 20)
What is data integrity?  Data integrity ensures
the accuracy, consistency, and reliability of
data. It can be enforced through primary key
constraints, foreign key constraints, and data
validation rules. Remember, these questions and
answers serve as a guide for interview
preparation, but its always beneficial to have a
solid understanding of SQL concepts and be able
to explain them in your own words. Good luck with
your interviews!