SQL | Foreign Key

A foreign key is a key used to link two tables. 

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 a real database table. we can create a view by selecting fields from one of the tables present in the database. A View can have all the rows of a table or specific rows based on certain conditions.

SQL | Difference between primary key and unique constraints

There is only one primary key in a table. It creates the clustered index automatically and it cannot have NULL value
whereas there can be multiple unique constraints in a table. It does not create the clustered index automatically and it can have a NULL value

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.

SQL | difference between BETWEEN and IN operator

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

MySQL | Repair Table

REPAIR command is used for repair a MySQL table

REPAIR TABLE users

We can also specify Quick and Extended after the table name

REPAIR TABLE users QUICK

If we specify Quick then MySQL will do a repair of only the index tree.

REPAIR TABLE users EXTENDED

 But if we specify the extended then it creates index row by row.

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

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

DATEDIFF(‘2018-03-07′,’2017-01-01’)

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.

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

MySQL | Find the number of rows in a table

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

>SELECT COUNT(*) FROM table_name;

MySQL |  Increase the speed of a MySQL select query

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

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

MySQL | What is the best collation to use for MySQL?

Mostly we use:

• utf8_general_ci
• utf8_unicode_ci

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

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

Mysql | Trigger

The trigger can be defined as a database object just like a stored procedure. It is a set of actions that fires automatically when an event occurs in a database.