The select command of SQL is used to retrieve data from the tables of the database and display those data
in a table format.
The capabilities of SQL select statements are as mentioned below
1. Projection
2. Selection
3. Join
The
general form of select statement is
shown below.
SELECT *|{[DISTINCT] column|expression [alias],...}
FROM tableName;
|
In
above general form:
1. The SELECT indentifies which column to select from the table specified.
2. The FROM indentifies which table to chose from the database.
Now lets’ analyze the statement SELECT *|{[DISTINCT] column|expression [alias],...}, first we will start
with the ‘ * ‘ operator.
The ‘ * ‘ operator in the SELECT statement:
When the ‘ * ‘ operator is used all the columns and
its respective values present in the table are retrieved. The below example
shows you the use of ‘ * ‘ operator.
select * from departments;
|
Here, all the
columns and its respective data from the departments table is retrieve and displayed as shown in the below table image.
The ‘ DISTINCT ‘
keyword used in the SELECT statement:
The DISTINCT keyword is used to eliminate the duplicate rows in the result of the select statement.
Consider the
below query which is written without DISTINCT
keyword.
select department_id
from employees;
|
You can see the output where the ‘ department_id ‘ column displays many
rows with duplicate values.
Now consider the same query where duplicate rows are eliminated using DISTINCT keyword.
select DISTINCT
department_id
from employees;
|
In the result you can see all the duplicate values displayed in previous result is eliminated, only the distinct values are displayed.
