Most Essential And Frequently Asked Interview Questions And Answer


SQL | difference between BETWEEN and IN operator

The BETWEEN operator select all records between the given range whereas IN operator select all matching records  given with WHERE clause.

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The three different way in PHP to connect with MySQL is:

  1. MySQL
  2. MySQLI
  3. PDO

Note: We should use MySQLI because MySQLI is imporoved version of MySQL and have more function then MySQL.


SQL | View

View in SQL is a kind of virtual table. It does not physical exists in our database but have rows and column as there we have in real database table. we can create a view by selecting fields from one or tables present in the database. A View can have all the rows of a table or specific rows based on certain condition.

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OPTIMIZE command is use for optimize a table.




SQL | LIKE operator

The LIKE operator of SQL is used for such kind of operations. It is used to fetch filtered data by searching for a particular pattern in where clause.

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We can use bellow query to count number of rows in a table

 >SELECT COUNT(*) FROM table_name;



Bellow code is use for the same

 $sql = "SELECT * FROM table1";
$result = mysql_query($sql, $db_link);
$num_rows = mysql_num_rows($result);
echo $num_rows;



For increaseing the speed of a query we have to do couple of things

  • First of all instead of using select * from table1, use select column1, column2, column3.. from table1.
  • Try to introduce index in the table.
  • Always use limit keyword if we are looking for any specific number of rows from the result set.

Mostly we use:

  • utf8_general_ci
  • utf8_unicode_ci

utf8_general_ci is use for fast sorting but it is less accurate whereas utf8_unicode_ci is used for better accuracy but little slow as compare to utf8_general_ci.

We can also use other collation based on specific languages like utf8_swedish_ci.


There are 5 different types of storage engine present in MySql.

  1. MyISAM
  2. Heap
  3. Merge
  4. INNO DB
  5. ISAM

MyISAM is the default storage engine for MySQL


SQL | Difference between primary key and unique constraints

There is only one primary key in a table. It creates the cluster index automatically and it cannot have NULL value
where as there can be multiple unique constraints in a table. It dose not creates the cluster index automatically and it can have NULL value


DATEDIFF function is use for finding the number of days between two given dates using MySQL




By using mysql function mysqli_insert_id() we can retrive the last inserted id in database table.



Trigger can be define as a database object just like a stored procedure. It is a set of actions which fires automatically when an event occure in a database.

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By using following query we can find out the nth largest salary from employees table.

 SELECT salary FROM `employees` ORDER BY salary DESC LIMIT 1 OFFSET n-1

In the above query n is replaced by any number where n >= 1. Suppose you want to find fifth highest salary then you have to use offset = 5-1 i.e.: 4

The query will be:

 SELECT salary FROM `employees` ORDER BY salary DESC LIMIT 1 OFFSET 4