Preparing a candidate for a SQL test can be a daunting prospect. To ensure they are adequately prepared, it is essential to provide them with a comprehensive set of SQL interview questions and solutions.
Hiring managers and job-seekers alike may benefit from this collection of SQL interview questions covering a wide range of experience levels.
SQL Interview Questions for Novices
Let’s get started by discussing some common SQL interview questions for new candidates.
What exactly is a VIEW?
In SQL, a “view” is a fictitious table that may be used to compile data from several sources (often other databases) into a single, useful whole.
The resulting view may be used in the same manner as any other database table:
The DROP VIEW command may be used to remove a view:
The difference between a primary key and a foreign key.
A Primary Key is a column in a table which must not be empty and must contain a value that is unique across all other records in the database. There can only be one Primary Key for any given table.
For the sake of maintaining referential integrity, a column may be labelled as a FOREIGN KEY if it references a PRIMARY KEY in another table. Unlike a PRIMARY KEY, multiple FOREIGN KEYs allow for duplicate data to be stored in a table.
Can you explain the difference between DELETE and TRUNCATE?
The DELETE command is used to remove rows from a table or relation, which is a Data Manipulation Language (DML) operation. All rows without a matching WHERE clause will be deleted. Furthermore, the DELETE command records the rows that have been removed, allowing for a rollback if needed.
The Data Definition Language (DDL) command TRUNCATE removes all rows from a table without warning or confirmation, making it potentially faster than a DELETE command, which must evaluate criteria. It is important to note that TRUNCATE cannot be undone as it does not record the rows that have been deleted.
Can you explain the meaning of the word “DEFAULT”?
The DEFAULT keyword can be used to set a column’s default value in the event that no value is provided. This restriction will only affect data that is entered into the system from this point onwards.
Values for the system may also be inserted using the DEFAULT keyword
What exactly is normalisation?
When normalising a database, its data is organised in tables with the aim of eliminating unnecessary information and simplifying the structure.
There are several stages to this process, each of which is a refinement of the last and aims to solve and avoid more issues than the one before it.
- Normal Form One (1NF)
- Forma Normale No. 2 (2NF)
- Forma Normale Tercera (3NF)
- The Normal Boyce-Codd Form (BCNF)
Questions for an Intermediate-Level SQL Interview
Here we provide you with some more complex SQL Server interview questions to help you prepare.
Just what are the distinctions between Clustered and Non-Clustered Indexes?
When applied to a specific table, a clustered index alters the structure of the table in memory. Generally, the primary key of a table also serves as the one and only clustered index.
A non-clustered index stores its data in a separate location, rather than being integrated into the database. A duplicate of the table is then created with the data organised in a way that can improve the performance of frequently-run or certain queries. It is possible to have multiple non-clustered indexes on a single table.
Collation is defined as.
In SQL server-related job interviews, it is common to be asked about the rules governing data storage, retrieval and comparison in a SQL database, known as ‘collation’.
You may either be case sensitive or case insensitive, but those are the two most frequent settings.
A database-wide or even per-table collation definition is possible.
Let’s pretend we have the following information in a table:
Group at table:
| ID | Name |
| 1 | Evelyn |
| 2 | Jonatan |
| 3 | Rick |
When we run the following search:
Case insensitive collations would return the first row of the table, whereas case sensitive collations would return an empty result set.
I’m confused about the distinction between a primary key and a unique key.
A primary key is a column of a table that must contain a value that is unique among all other records in the database, and must not be null. Having only one primary key per table is contrary to a clustered index.
Columns containing a single value are identified by the UNIQUE key. This value may be blank (though it can only occur once). Multiple UNIQUE keys can be added to a table without compromising its ability to operate as a non-clustered index.
Can you explain the distinction between an INNER JOIN, an OUTER JOIN, and a FULL OUTER JOIN?
One of the most used join types, INNER JOIN will only return rows if the join condition is met.
When employing an OUTER JOIN, all rows from the primary table (the left table in a LEFT OUTER JOIN, and the right table in a RIGHT OUTER JOIN) will be matched with rows from the target table. If a suitable value cannot be identified, a NULL value will be used to fill the void.
The operation of FULL OUTER JOIN is equivalent to using both LEFT OUTER JOIN and RIGHT OUTER JOIN together.
Take a look at this illustration:
Group at table:
| ID | Name | JobID |
| 1 | Evelyn | 2 |
| 2 | Jonatan | NULL |
| 3 | Rick | 1 |
Employment 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 |
DISCERNING 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 |
How come there’s no clear distinction between UNION and UNION ALL?
UNION and UNION ALL both combine the outcomes of two SELECT statements, but they do it in different ways.
- UNION merges the results without displaying duplicate values.
- By using UNION ALL, you may combine all of the values, even those that appear more than once.
Here’s a case in point:
Database of births:
| 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 |
CALL TO UNION:
| Name | Country |
| Rick | USA |
| Ardeth | Egypt |
| Rick | Egypt |
| Ardeth | Egypt |
Excelent SQL Questions for Your Next Job Interview
This section provides challenging questions to ask during your SQL interview for senior engineers.
Exactly what are the variations between Shared Lock, Exclusive Lock, and Update Lock?
Any amount of processes can utilise a shared lock to access the data stored in a resource, however no other alterations can be made to this information.
An exclusive lock prevents any other processes from reading from or writing to the locked resource.
An update lock is an exclusive lock that permits some flexibility. It can be used to modify an existing shared lock, and will switch to an exclusive lock at the time of the transaction, thus preventing any further access to the data. The process of changing from shared to exclusive status has been likened to a “dynamic lock”.
Exactly how unlike are the operations of UNION, MINUS, and INTERSECT?
The output of two SELECT queries may be combined using the UNION, MINUS, and INTERSECT operators; the distinction between them is in how they do it.
- UNION merges the results without displaying duplicate values.
- By using MINUS, you may get the result of a SELECT query subtraction.
- With INTERSECT, you’ll only see rows that match both SELECTs.
Here’s a case in point:
Database of births:
| 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 |
MINUS:
| Name | Country |
| Rick | USA |
INTERSECT:
| Name | Country |
| Ardeth | Egypt |
Which is quicker, a single large query or a large number of tiny ones?
The answer to this question is conditional.
In general, a large query is likely to run more efficiently as there is less overhead with each connection to the database. However, the time saved by reducing the number of database connections could be cancelled out by the time taken to transmit data that is not relevant to the current task.
It may be more efficient to consolidate multiple, independent inquiries into one larger one if their outcomes are interdependent. However, if the inquiries can be executed in parallel, the results may be obtained more quickly by leaving them separate.
Explain the variations between B-Tree, R-Tree, and Hash indexing.
Each of these indexes shines in a certain area of comparison, making it crucial to choose the right one for quick searches:
- B-Trees are a highly efficient form of data storage that facilitates quick searching and sorting of records for speedy retrieval. Furthermore, these trees enable users to rapidly identify values within a specified range.
- To quickly retrieve records with values close to a specific value across two or more dimensions, an R-Tree spatial index can be utilised. This is a common feature seen in geographic information systems.
- This is a key-value map with no specific order to the keys or values. It is more efficient than B-Trees when attempting to retrieve records, however, due to the absence of an ordering system, it cannot be used to retrieve a range of items.
Both “optimistic locking” and “pessimistic locking” are explained.
Locking in Hope This approach allows for multiple users to access the same record in a database, however only the most up-to-date version can be written to. This strategy utilises timestamps or version numbers to ensure that the most recent version of the record is being used. If an issue arises, the action is reversed.
Optimistic LockingHowever, restricts access to a record to only one user until that person no longer needs it.
Top SQL Interview Questions for Recruiting Top-Notch Distributed Programmers
By utilising the questions posed in the article, you can quickly evaluate the applicant’s expertise. Alternatively, you may consider hosting a coding contest whereby contenders tackle a stimulating, yet difficult, subject. This way, you can identify the optimal candidate for the job in a shorter period of time.
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