The task of getting a candidate ready for a SQL test can seem overwhelming. It is crucial to equip them with a thorough selection of SQL interview questions and answers to ensure they are well-prepared.
This compilation of SQL interview questions and answers caters to a variety of levels of expertise and can be helpful for both recruiters and job seekers.
SQL Interview Questions for Beginners
To begin with, we will explore some typical SQL questions interview for fresh candidates.
What is the Definition of a VIEW in SQL?
A “view” in SQL is an artificial table created to consolidate information from multiple sources, such as other databases, into a unified and useful format.
Once created, the resultant view can be utilized like any other table in a database:
Eliminating a view is possible with the DROP VIEW command:
Distinguishing Between Primary Key and Foreign Key
A Primary Key is a column in a table that cannot be left blank and must possess a value that is exclusive compared to all other records in the database. A table may only have a single SQL primary key.
To maintain referential integrity, a column can be designated as a FOREIGN KEY if it references a PRIMARY KEY from a different table. Unlike a PRIMARY KEY SQL, a table can have multiple FOREIGN KEYs SQL, which allows for duplicate data to be stored.
Could You Elaborate on the Distinction between DELETE and TRUNCATE Commands?
The DELETE command is utilized to erase rows from a table or relation, which is a Data Manipulation Language (DML) operation. It only removes the rows that fit within the given WHERE clause condition. Delete command in SQL also maintains a record of the deleted rows, allowing for a rollback to take place if necessary.
The TRUNCATE command, classified as a Data Definition Language (DDL) operation, removes all rows from a table without warning or confirmation, making it potentially faster than DELETE, which needs to evaluate conditions. The truncate SQL command is critical to remember that TRUNCATE cannot be undone since it does not record the deleted rows.
Could You Elaborate on the Significance of the Term “DEFAULT” in SQL?
In SQL, the DEFAULT keyword may be employed to specify the default value of a column if no values are given. This guideline only affects new data entered into the system from this point on.
The DEFAULT keyword can also be used to insert values into the system.
What is Normalization in SQL
The normalization process is composed of several stages, each one refining the previous stage and aiming to solve and prevent more problems than its predecessor.
Normalization in SQL involves structuring database information into tables to eliminate redundant data and simplify the structure.
- First Normal Form (1NF)
- Second Normal Form (2NF)
- Third Normal Form (3NF)
- Boyce-Codd Normal Form (BCNF)
Intermediate-Level SQL Interview Questions
We’ve compiled a list of more advanced interview questions on SQL server to assist you in your preparation.
What are the Differences Between Clustered and Non-Clustered Indexes?
When applied to a particular table, a clustered index modifies the table’s structure in memory. Typically, a table’s primary key also serves as its only clustered index.
Unlike a clustered index, a non-clustered index stores its data in a separate location, rather than being integrated into the database. A copy of the table is created with the data arranged in a manner that may improve the performance of certain or frequently-executed queries. A single table can have multiple non-clustered indexes.
What is Collation?
During SQL Server job interviews, one may frequently be questioned about the guidelines for storing, accessing, and comparing data in a SQL database, which are known as ‘collation’.
The two most common settings are case sensitivity and case insensitivity.
It’s possible to define collation at the database-wide or even per-table level.
Suppose we have the following data in a table:
Grouped by table:
| ID | Name |
| 1 | Evelyn |
| 2 | Jonatan |
| 3 | Rick |
If we execute the following query:
Queries with case-insensitive collations would retrieve the first row of the table, while case-sensitive collations would return an empty result set.
What is the difference between a primary key and a unique key?
A primary key is a column in a table that must have a value that is distinct from all other records in the database and cannot be null. Having more than one primary key per table goes against the clustered index.
Columns that contain only one distinct value are identified by the UNIQUE key. This value can be empty, but it must be unique. We can add multiple UNIQUE keys to a table without sacrificing its ability to function as a non-clustered index.
What is the difference between an INNER JOIN, an OUTER JOIN, and a FULL OUTER JOIN?
One of the most frequently used join types, an INNER JOIN only yields rows that satisfy the join condition.
In an OUTER JOIN, all records of the primary table (i.e., the left table in a LEFT OUTER JOIN, and the right table in a RIGHT OUTER JOIN) will combine with rows of the matching table. If there is no appropriate value, a NULL value will be inserted.
The FULL OUTER JOIN operation is the same as using LEFT OUTER JOIN and RIGHT OUTER JOIN jointly.
Refer to this illustration:
Group by Table:
| ID | Name | JobID |
| 1 | Evelyn | 2 |
| 2 | Jonatan | NULL |
| 3 | Rick | 1 |
Job Opportunities at the Table:
| ID | Name |
| 1 | Colonel |
| 2 | Librarian |
| 3 | Medjai |
Connection Within:
| People.ID | People.Name | People.JobID | Jobs.ID | Jobs.Name |
| 1 | Evelyn | 2 | 2 | Librarian |
| 3 | Rick | 1 | 1 | Colonel |
Distinctive Part:
| ID | Name | JobID | JobName |
| 1 | Evelyn | 2 | Librarian |
| 2 | Jonatan | NULL | NULL |
| 3 | Rick | 1 | Colonel |
| People.ID | People.Name | People.JobID | Jobs.ID | Jobs.Name |
| 1 | Evelyn | 2 | 2 | Librarian |
| 2 | Jonatan | NULL | NULL | NULL |
| 3 | Rick | 1 | 1 | Colonel |
Continuous Joining:
| People.ID | People.Name | People.JobID | Jobs.ID | Jobs.Name |
| 1 | Evelyn | 2 | 2 | Librarian |
| 2 | Jonatan | NULL | NULL | NULL |
| 3 | Rick | 1 | 1 | Colonel |
| NULL | NULL | NULL | 4 | Medjai |
Why is there a lack of clear differentiation between UNION and UNION ALL?
UNION and UNION ALL blend the results of two SELECT statements, but they accomplish it in distinct ways.
- UNION combines the results while eliminating duplicate values.
- UNION ALL enables combining all values, even if they appear more than once.
Here’s an example:
Births Database:
| ID | Name | Country |
| 1 | Rick | USA |
| 2 | Ardeth | Egypt |
LifeRegistry Table:
| ID | Name | Country |
| 1 | Rick | Egypt |
| 2 | Ardeth | Egypt |
UNION:
| Name | Country |
| Rick | USA |
| Ardeth | Egypt |
| Rick | Egypt |
Calling UNION:
| Name | Country |
| Rick | USA |
| Ardeth | Egypt |
| Rick | Egypt |
| Ardeth | Egypt |
Great SQL Questions for Your Upcoming Job Interview
This segment offers tough questions to pose to senior engineers during their SQL job interviews.
What are the differences between Shared Lock, Exclusive Lock, and Update Lock?
Multiple processes can use a shared lock to access the data in a resource, but no other modifications can be performed on this data.
Exclusive lock bars other processes from reading or writing to the locked resource.
An update lock is an exclusive lock that offers some flexibility. It can modify an existing shared lock and can switch to an exclusive lock during the transaction to prevent further access to the data. The change from shared to exclusive status is comparable to a “dynamic lock”.
How different are the functionalities of UNION, MINUS, and INTERSECT?
To combine the results of two SELECT queries, we can use the UNION, MINUS, and INTERSECT operators, which vary in how they perform the task.
- UNION combines the outcome while suppressing duplicate values.
- MINUS is used to subtract the result of a SELECT query.
- INTERSECT displays only the matching rows from both SELECT queries.
For instance:
Birth database:
| ID | Name | Country |
| 1 | Rick | USA |
| 2 | Ardeth | Egypt |
Table of LifeRegistry
| ID | Name | Country |
| 1 | Rick | Egypt |
| 2 | Ardeth | Egypt |
UNION:
| Name | Country |
| Rick | USA |
| Ardeth | Egypt |
| Rick | Egypt |
MINUS:
| Name | Country |
| Rick | USA |
INTERSECT:
| Name | Country |
| Ardeth | Egypt |
Is it more efficient to use a single large query or many small queries?
The answer depends on various factors.
Usually, a single large query is more efficient as it reduces the overhead of connecting to the database multiple times. However, the time saved from minimizing the number of database connections may be negated by the time it takes to transmit irrelevant data for the task at hand.
If the results of multiple independent queries are interdependent, it may be more efficient to combine them into one larger query. On the other hand, if the queries can be executed in parallel, it might be quicker to leave them as separate queries.
Describe the differences among B-Tree, R-Tree, and Hash indexing.
Each of these indexes excels in specific criteria, making it essential to select the appropriate one for swift searches:
- B-Trees are an extremely effective data storage format for fast searching and sorting of records to retrieve data quickly. Additionally, these trees provide the ability to swiftly locate values within a particular range.
- To retrieve records swiftly with values within close proximity of a specific value across multiple dimensions, an R-Tree spatial index can be utilized. This functionality is frequently seen in geographic information systems.
- This is a key-value map with no particular sequence for the keys or values. It is more efficient than B-Trees when retrieving records; however, since there is no ordering mechanism, it cannot be employed to obtain a range of items.
Explanation of “optimistic locking” and “pessimistic locking”.
Optimistic Locking permits multiple users to access the same record in a database, but only the most recent version can be written. This technique employs timestamps or version numbers to verify that the most recent edition of the record is being used. If there is a problem, the process is undone.
Pessimistic Locking, however, allows access to a record to only one user until that user has completed using it.
Primary SQL Interview Questions to Hire Exceptional Distributed Programmers
By using the questions described in the article, you can promptly evaluate the applicant’s skills. Alternatively, you may choose to conduct a coding competition where candidates tackle a challenging yet intriguing topic. This method allows for the identification of the best candidate for the position in a shorter span of time.
At Works, we can assist you in locating the perfect SQL Developers for your development teams. Our vast pool of candidates ensures that we can discover the right match for your company. If you’re interested, kindly visit our job openings page.
