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# SQL - Expressions

An expression is a combination of one or more values, operators, and SQL functions that evaluate to a value.

SQL EXPRESSIONs are like formulas and they are written in query language. You can also used to query the database for specific set of data.

# Syntax:

Consider the basic syntax of the SELECT statement as follows:

    SELECT column1, column2, columnN
    FROM table_name
    WHERE [CONTION|EXPRESSION];

There are different types of SQL expression, which are mentioned below:

# SQL - Boolean Expressions:

SQL Boolean Expressions fetch the data on the basis of matching single value. Following is the syntax:

    SELECT column1, column2, columnN
    FROM table_name
    WHERE SINGLE VALUE MATCHTING EXPRESSION;

Consider CUSTOMERS table has following records:

    SQL> SELECT * FROM CUSTOMERS;
    +----+----------+-----+-----------+----------+
    | ID | NAME     | AGE | ADDRESS   | SALARY   |
    +----+----------+-----+-----------+----------+
    |  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
    |  2 | Khilan   |  25 | Delhi     |  1500.00 |
    |  3 | kaushik  |  23 | Kota      |  2000.00 |
    |  4 | Chaitali |  25 | Mumbai    |  6500.00 |
    |  5 | Hardik   |  27 | Bhopal    |  8500.00 |
    |  6 | Komal    |  22 | MP        |  4500.00 |
    |  7 | Muffy    |  24 | Indore    | 10000.00 |
    +----+----------+-----+-----------+----------+
    7 rows in set (0.00 sec)

Here is simple examples showing usage of SQL Boolean Expressions:

    SQL> SELECT * FROM CUSTOMERS WHERE SALARY = 10000;
    +----+-------+-----+---------+----------+
    | ID | NAME  | AGE | ADDRESS | SALARY   |
    +----+-------+-----+---------+----------+
    |  7 | Muffy |  24 | Indore  | 10000.00 |
    +----+-------+-----+---------+----------+
    1 row in set (0.00 sec)

# SQL - Numeric Expression:

This expression is used to perform any mathematical operation in any query. Following is the syntax:

    SELECT numerical_expression as  OPERATION_NAME
    [FROM table_name
    WHERE CONDITION] ;

Here numerical_expression is used for mathematical expression or any formula. Following is a simple examples showing usage of SQL Numeric Expressions:

    SQL> SELECT (15 + 6) AS ADDITION
    +----------+
    | ADDITION |
    +----------+
    |       21 |
    +----------+
    1 row in set (0.00 sec)

There are several built-in functions like avg(), sum(), count() etc.to perform what is known as aggregate data calculations against a table or a specific table column.

    SQL> SELECT COUNT(*) AS "RECORDS" FROM CUSTOMERS;
    +---------+
    | RECORDS |
    +---------+
    |       7 |
    +---------+
    1 row in set (0.00 sec)

# SQL - Date Expressions:

Date Expressions return current system date and time values:

    SQL>  SELECT CURRENT_TIMESTAMP;
    +---------------------+
    | Current_Timestamp   |
    +---------------------+
    | 2009-11-12 06:40:23 |
    +---------------------+
    1 row in set (0.00 sec)

Another date expression is as follows:

    SQL>  SELECT  GETDATE();;
    +-------------------------+
    | GETDATE                 |
    +-------------------------+
    | 2009-10-22 12:07:18.140 |
    +-------------------------+
    1 row in set (0.00 sec)

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