SQL Interview Questions and Answers
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Best SQL Interview Questions and Answers
Here you will find the most frequently asked top 50 SQL Interview Questions and Answers which will help the aspirants preparing for the SQL interviews to boost their SQL intelligence. These questions are prepared in consultation with the top interviewers. We have covered almost all the topics that are in association with SQL. These top 50 SQL interview questions and answers will be certainly useful for the aspirants to crack the SQL interview. SQL is now highly used in major multinational companies and so there is a huge demand for certified SQL professionals.
Are you now preparing for a SQL interview? Then these top 50 SQL interview questions and answers will be your cup of coffee. Most of the employers have asked these questions in the interview to evaluate the skill set of the aspirants. Keep up your confidence and attend any of the SQL interviews, just prepare these top 50 SQL interview questions and answers and boost your future career. We wish you all success in your future endeavor.
Top SQL Interview Questions and Answers
Structured Query Language or SQL is effectively utilized to communicate with the databases. As per ANSI, it is stated that SQL can also be defined as the Structured Query Language for Relational Database Management Systems (RDBMS) which can be used to maintain and perform various data manipulation operations. Generally, SQL is also a database language which can be used to create and delete databases and also to modify and fetch table rows for several other necessities.
A database is generally a collection of structured data while DBMS or Database Management System is a well-organized software application that interacts with the applications, users and the database itself to analyze and capture data. These data can be actually modified, retrieved and deleted and these data can be of any types which include numbers, strings, images and more. The different types of DBMS are Relational Database Management Systems and Non-Relational Database Management Systems.
SQL | MySQL |
SQL is generally Structured Query Language and it is a standard language too | MySQL is referred to as a database management system |
SQL is used to access and manage database as it is a core relational database | MySQL is also RDMS, a Relational Database Management Systems which include SQL Server, Informix and more |
SQL | NoSQL |
It is a relational database | It is a non-relational database or it can also be denoted as distributed database |
There are several tools present for performance analysis | It doesn’t comprise of relevant tools to analyze data and the performance |
SQL is not actually suitable for hierarchical data storage | NoSQL is highly suitable for hierarchical data storage |
The different types of subset present in SQL are as follows:
- Data Definition Language (DDL)
- Data Manipulation Language (DML)
- Data Control Language (DCL)
- Transaction Control Language (TCL)
DDL or Data Definition Language is a part of SQL that describes the data structure of the database at the initial stage while the database is to be created. DDL can be utilized in creating and restructuring the objects of the database. The commands involved in DDL or Data Definition Language are as follows:
- Create table
- Alter table
- Drop table
To retrieve and manipulate the existing data present in database, DML or Data Manipulation Language is used. The commands involved in DML are Insert, Update and Delete.
DCL can be used to control the data access in the database. It is actually used to create objects in line with user access and to grant access rights among several users. Some of commands involved in Data Control Language are Grant and Revoke.
The changes made by DML commands are controlled by TCL or Transaction Control Language and the commands used in TCL are Begin, Transaction, Commit, Rollback and Savepoint.
A table can be explained as a collection of organized data which can be in the form of rows and columns while a field in SQL can be referred to as the number of columns available in a table.
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Joins in SQL is used to merge rows of two or more tables with identical columns. It can also be used to merge and retrieve data. The four types of joins in SQL are Inner join, Right join, Left join and Full join or Full Outer join.
Inner Join – If the join condition is satisfied, it returns all the rows of several tables given.
Left Join – Once the join condition is satisfied, it returns all the rows present in the left table while only the matching rows from the right table.
Right Join – Once the join condition is satisfied, it returns all the rows present in the right table while only the matching rows from the left table.
Full Join or Full Outer Join – If any match is found in any of the given tables, it returns all the rows present in both the left and right hand side of the table.
SQL | Oracle |
SQL is highly secure and scalable when compared to Oracle | Oracle is also secure and scalable but not as SQL |
Procedural extension is highly supported by SQL | Oracle doesn’t support Procedural extension as SQL |
Both are character data types where CHAR is used for strings with fixed length and VARCHAR2 is used for string with variable length.
Constraints in SQL are nothing but the limit applied to a particular data type present in a table. These constraints cam be set while creating or altering the statements of the table. Some of the examples of constraints are as follows:
- Not Null
- Check
- Default
- Unique
- Primary Key
- Foreign Key
The various levels of constraints are as follows:
- Column level constraint
- Table level constraint
The default constraint sets a default value for the column such that if none of the values are specified then the value assigned by the default constraint will be added to all the new records.
When all the values allocated to the columns are different then it can be described as the unique constraint.
A primary key is either a column or a collection of column which identifies each row of the table uniquely. Null values cannot be added or it is not allowed in primary key.
Foreign key present in the child table actually points out the primary key present in the parent table. It actually enforces a prime link between the data of both the table and maintains referential integrity. It also controls the activities that would terminate the bond between the parent and the child table.
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Data Integrity actually denotes the consistency and accuracy of the data that has been stored in the database.
DELETE | TRUNCATE |
It is actually used to delete a row in the table | It actually deletes all the rows in a table |
It comes under DML Command | It comes under DDL Command |
It is slower when compared to TRUNCATE statement | It is very fast |
DROP statement actually removes a table and it can never be rolledback again from the database while TRUNCATE statement deletes or removes all the rows present in a table.
WHERE | HAVING |
It is implemented in ROW operations | It is implemented in COLUMN operations |
It can be used to fetch a specific data from a specific row | It fetches the entire data and separates it according the given condition |
It can be used with SELECT, UPDATE and DELETE statements | It cannot be used without a SELECT statement |
To search for something at high speed in a database, index can be used. In case, if index is not present, then SQL server has to go through the entire table and all the rows to find the exact match, this might take a lot of time.
Some of the index types are as follows:
- Single column index
- Composite column index
- Unique index
Single column index is actually created for only one of the table columns.
Syntax:
CREATE INDEX index_name
ON table_name(column_name);
Composite column index is actually created for two or more columns of the table.
Syntax:
CREATE INDEX index_name
ON table_name (column1, column2)
Unique index actually assists to maintain data integrity of the table and it does not allow more values to be added in the table.
Syntax:
CREATE UNIQUE INDEX index
ON table_name(column_name)
Clustered Index | Non-clustered Index |
Faster retrieval of data from the database | Slower in retrieving data from the database |
A table can consist of only one clustered index | Any number of non-clustered index can be found in a table |
It alters the manner in which the records are stored in the database | It cannot alter the manner in which the records are stored in the database |
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GetDate() is one of the in-built functions of SQL which can be used to display the current date or timestamp.
The process of organizing data to avoid redundancy and duplication is known as normalization. Some of the advantages of normalization are as follows:
- It accesses data more efficiently
- It implements security at ease
- It ensures easy modification
- It is a highly compact database
- It searches for any particular information more quickly
The process of accessing data from the higher to the lower database forms are known as denormalization. It introduces redundancy into the table and thus assists the database managers to improve the overall performance of the infrastructure.
Trigger is a type of stored procedures which can be used to automatically execute in place or after data modifications. Trigger can also execute a particular batch of code when queries like insert, update or more are executed against a specific table.
Listed below are some of the different operators found in SQL:
- Arithmetic operators
- Logical operators
- Comparison operators
The different types of set operators found in SQL are union, Intersect and Minus operator.
No, a NULL value is not always identical to zero or a blank space. NULL value is generally a value that is unavailable, unknown or not assigned or not applicable while zero is a number and a blank space is a character.
Cross join actually generates cross product or Cartesian product of two tables while natural join is when all the columns have the same name and data type present in both the tables.
A query inside another query which is used to retrieve data or get back the information from a database is known as a subquery in which the outer query is called the main query whereas the inner query is termed as the subquery. In most of the conditions, the subqueries are executed first and the results are passed to the main query.
There are different types of subquery and they are correlated subquery and non-correlated subquery.
SELECT * FROM Table_name WHERE EmpName like ‘A%’
The command described above will be used to display the employee names that start with ‘A’.
Group functions in SQL execute on a particular set of row and return a single result per group. The most commonly used group functions are AVG, COUNT, MAX, MIN, SUM, and VARIANCE.
The common connection between the tables in a database is known as relationships. These relations or links are found between entities which have something to provide and perform between the both.
The different types of relationships found in SQL are as follows:
- One to One Relationship
- One to Many Relationship
- Many to One Relationship
- Self-Referencing Relationship
BETWEEN operators are used to display specific that are in line with a specific range of row values while IN operator is used to check a specific value that are present in a specific set of values.
It ensures updation or insertion of a data into a particular table. Update occurs when a row exists and Insert occurs when a row does not exist.
SQL functions are widely used because:
- It can easily modify individual data items
- It can easily convert the data types
- It can easily manipulate the output
- It can easily perform calculations on any complex data
- It can easily format dates and numbers
Listed below are some of the ways through which dynamic SQL can be executed:
Initially write an effective query with the parameters with the help of EXEC. and sp_executesql.
ALIAS name can be assigned to any column or a table which can be referred in a WHERE clause to analyze a particular column or a table.
Lower, Upper and Initcap are some of the manipulation functions found in SQL.
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