DataBase--Count & Group by

SQL Count () function:

• SQL count (column_name) syntax: The count (column_name) function returns the number of rows that are worth the specified column (null does not count in)

Select Count  from table_name

• SQL count (DISTINCT column_name) Syntax: The count (DISTINCT column_name) function returns the number of different worth of the specified column

Select Count (distinct from table_name

• SQL COUNT (*) Syntax: the COUNT (*) function returns the number of records in a table

Select Count (* from table_name

SQL GROUP by statement:

Aggregate functions, such as the SUM function, are often required to add a group by statement.

Grammar:

SELECT column_name, Aggregate_function (column_name)  from table_name WHERE column_name operator Value GROUP  by column_name

Instance:

Question:

Write a SQL query to the Find all duplicate emails in a table named Person .

+----+---------+| Id | Email   |+----+---------+| 1  | [Email protected] | | 2  | [Email protected] | | 3  | [Email protected] |+----+---------+

For example, your query should return the following for the above table:

+---------+| Email   |+---------+| [Email protected] |+---------+

Note:all emails is in lowercase.

Analysis:

Write an SQL statement to find all the duplicate emails in the person table

Answer:

Select  from Person P1 Group  by  having Count (*>1;

DataBase--Count & Group by