SQL expression

An expression is one or more values 、 Operators and SQL Combination of functions . Every expression has a value , Through evaluation, we can get .

SQL Expressions look like mathematical formulas , They are written in query language . You can also use them to query the database for data that meets specific conditions .

grammar

Consider the following SELECT Basic syntax of statements ：

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

SQL There are many different types of expressions , As shown below ：

Boolean expression

SQL Boolean expressions retrieve data in a single valued condition , The syntax is as follows ：

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

Consider the customer information table shown below ：

    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's a show SQL A simple example of a Boolean expression ：

    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)

Numerical expression

Numeric expressions are used to perform mathematical operations in queries . The syntax is as follows ：

    SELECT numerical_expression as  OPERATION_NAME
    [FROM table_name
    WHERE CONDITION] ;

here numerical_expression For mathematical operations or any formula . Here is a description SQL A simple example of the use of numerical expressions ：

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

SQL There are a series of built-in functions , for example avg()、sum()、count() etc. , These functions are used to perform aggregate data operations on a table or on specific columns in a table .

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

Time expression

The time expression returns the date and time of the current system ：

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

Here is a date expression ：

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