Saturday, 19 November 2011

SQL SELECT Statement

The most commonly used SQL command is SELECT statement. The SQL SELECT statement is used to query or retrieve data from a table in the database. A query may retrieve information from specified columns or from all of the columns in the table. To create a simple SQL SELECT Statement, you must specify the column(s) name and the table name. The whole query is called SQL SELECT Statement.

Syntax of SQL SELECT Statement:

SELECT column_list FROM table-name 
[WHERE Clause]
[GROUP BY clause]
[HAVING clause]
[ORDER BY clause];
  • table-name is the name of the table from which the information is retrieved.
  • column_list includes one or more columns from which data is retrieved.
  • The code within the brackets is optional.
database table student_details;
NOTE: These database tables are used here for better explanation of SQL commands. In reality, the tables can have different columns and different data.
For example, consider the table student_details. To select the first name of all the students the query would be like:
SELECT first_name FROM student_details;
NOTE: The commands are not case sensitive. The above SELECT statement can also be written as "select first_name from students_details;"
You can also retrieve data from more than one column. For example, to select first name and last name of all the students.
SELECT first_name, last_name FROM student_details;
You can also use clauses like WHERE, GROUP BY, HAVING, ORDER BY with SELECT statement. We will discuss these commands in coming chapters.
NOTE: In a SQL SELECT statement only SELECT and FROM statements are mandatory. Other clauses like WHERE, ORDER BY, GROUP BY, HAVING are optional.

How to use expressions in SQL SELECT Statement?

Expressions combine many arithmetic operators, they can be used in SELECT, WHERE and ORDER BY Clauses of the SQL SELECT Statement.
Here we will explain how to use expressions in the SQL SELECT Statement. About using expressions in WHERE and ORDER BY clause, they will be explained in their respective sections.
The operators are evaluated in a specific order of precedence, when more than one arithmetic operator is used in an expression. The order of evaluation is: parentheses, division, multiplication, addition, and subtraction. The evaluation is performed from the left to the right of the expression.
For example: If we want to display the first and last name of an employee combined together, the SQL Select Statement would be like
SELECT first_name || ' ' || last_name FROM employee;
first_name || ' ' || last_name
Rahul Sharma
Anjali Bhagwat
Stephen Fleming
Shekar Gowda
Priya Chandra
You can also provide aliases as below.
SELECT first_name || ' ' || last_name AS emp_name FROM employee;
Rahul Sharma
Anjali Bhagwat
Stephen Fleming
Shekar Gowda
Priya Chandra


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