TOP-5 Interview Questions in SQL for Beginners

SQL stands for Standard Query Language. SQL is primarily used to manage the Database Management System data. There is a very high demand for SQL developers in the market, and if you are going to appear for an interview, you have landed in the right place. The following questions are the most likely asked Interview questions in SQL. Please go through them and ace your interview with flying colors.

1. What is the difference between SQL and MySQL?

SQL stands for Standard query language. It is used as a standard language to manage, access, and interact with huge databases. With SQL, you can alter the code as per the requirement, and the results will be shown in the form of relations ( or table). On the other hand, MySQL is an open-source relational database management system used worldwide.

So, the following are the differences between SQL and MySQL:

1. SQL is a query language, whereas MySQL is a relational database upon which queries can be carried out with SQL.
2. MySQL stores alter or modifies, and manages data in tabular format, whereas SQL is used to retrieve data from such databases by writing queries.
3. SQL can support a single-engine, whereas MySQL can support multiple engines simultaneously.
4. The server of SQL remains independent of the database, whereas, during the data backup session, the server blocks the database.
5. SQL remains the standard language, with its format and syntax remaining pretty much the same after any update, whereas MySQL changes after frequent updates.

2. What are the different constraints in SQL?

The most commonly used constraints in SQL are as follows:

1. UNIQUE CONSTRAINT: This constraint makes sure that every entry in the column is different.
2. NOT NULL CONSTRAINT: This constraint doesn’t let you leave any column value NULL.
3. DEFAULT CONSTRAINT: If any column value is not assigned any value, a default value will be assigned.
4. PRIMARY KEY CONSTRAINT: This is a basic constraint that uniquely identifies each record or row of any table.
5. FOREIGN KEY CONSTRAINT: This constraint is used to build a link between two tables. It can be a combination of columns or one column whose value matches another table’s PRIMARY KEY.

3. What is Normalization, and why is it needed?

Normalization is a technique that helps in organizing the data in records/tables easily and efficiently. It also helps minimize redundancy and reduce anomalies like Update anomaly, Delete anomaly, insert anomaly, etc. Nevertheless, normalization also ensures that only related data is being stored in the tables.

There are various levels of normalization, which are as follows:

1. First Normal Form(1NF): In this normal form, every column should have atomic values, which means data should be in its most reduced form, and every row should be necessarily uniquely identified by PRIMARY KEY.
2. Second Normal Form(2NF): To have the second normal form in any table, it should be in the first normal form, and it should not have any partial dependency.
3. Third Normal Form (3NF): To have the third normal form in any table, it should be in the second normal form, and it should not have any transitive dependency.

There are 6 Normal Forms, but 3 NF is most widely used for the majority of purposes.

4. What are different kinds of SQL commands?

There are six types of SQL commands, which are as follows:

1. DDL Commands: DDL stands for data definition language. DDL commands let us alter or change the table’s whole structure, such as creating or deleting the table. Examples of DDL commands are CREATE, DROP, RENAME, ALTER, TRUNCATE, etc.
2. DML Commands: DML stands for data manipulation language. DML commands allow us to manipulate the data within objects of any relational database. Examples for DML commands are DELETE, INSERT, UPDATE, etc.
3. DQL Commands: DQL stands for Data Query Language. The SELECT command is the most widely used command from this category, which, along with other clauses like WHERE, IN, etc., is used to query against the modern relational databases.
4. DCL Commands: DCL stands for Data Control Language. It helps the administrator to grant or revoke access to different entities. Examples for DCL commands are GRANT, REVOKE, etc.
5. Tcl Commands: TCL stands for Transaction Control Language. These commands help you manage the transactions of the relational database.

5. What are joins in SQL?

If you want to combine data from 2 or more rows from 2 or more tables, you will use SQL joins. There are four types of SQL joins, and they are as follows:

1. Left Join
2. Right Join
3. Inner Join
4. Full Join

LEFT JOIN: – LEFT JOIN returns all the rows from the left side of the join and the matching rows from the right side of the join. Suppose there is no matching row on the right side of the join, and the resulting set will have NULL in those rows and columns. Another name of LEFT JOIN is LEFT OUTER JOIN.

Syntax:

SELECT table1.column1, table1.column2, table1.column3, table2.column1,....
FROM table1
LEFT JOIN table2
ON table1.matching_column = table2.matching_column;

Here the matching column is the common column of both tables.

RIGHT JOIN: – RIGHT JOIN returns all the rows from the right side of the join and the matching rows from the join’s left side. Suppose there is no matching row on the left side of the join, and the resulting set will have NULL in those rows and columns. Another name of RIGHT JOIN is RIGHT OUTER JOIN.

Syntax:

SELECT table1.column1, table1.column2, table1.column3, table2.column1,....
FROM table1
RIGHT JOIN table2
ON table1.matching_column = table2.matching_column;

INNER JOIN: – INNER JOIN combines all the rows and columns of the respective tables as long as the condition satisfies. So, in the resulting rows, the values of the common field will be the same.

Syntax:

SELECT table1.column1, table1.column2, table1.column3, table2.column1,....
FROM table1
INNER JOIN table2
ON table1.matching_column = table2.matching_column;

FULL JOIN: – FULL JOIN combines all the rows from both tables. It is the resulting set of LEFT JOIN and RIGHT JOIN together. Wherever there are no matching rows in the tables, the FULL JOIN set will have NULL values in those rows or columns.

Syntax:

SELECT table1.column1, table1.column2, table1.column3, table2.column1,....
FROM table1
FULL JOIN table2
ON table1.matching_column = table2.matching_column;

   

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