表达式是一个或多个值,运算符和SQL函数的组合,它们计算为值.这些SQL EXPRESSIONs就像公式,它们是用查询语言编写的.您还可以使用它们来查询数据库中的特定数据集.
请考虑SELECT语句的基本语法,如下所示;
SELECT column1,column2,columnN FROM table_name WHERE [CONDITION | EXPRESSION];
有不同类型的SQL表达式,下面提到去;
布尔
数字
日期
让我们现在详细讨论每一个.
SQL布尔表达式基于匹配单个值来获取数据.以下是语法 :
SELECT column1,column2,columnN FROM table_name WHERE SINGLE VALUE MATCHING EXPRESSION;
考虑具有以下记录的CUSTOMERS表 :
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)
下表是一个简单示例,显示各种SQL布尔表达式的使用 :
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)
这些表达式用于在任何查询中执行任何数学运算.以下是语法 :
SELECT numerical_expression as OPERATION_NAME [FROM table_name WHERE CONDITION];
这里,numeric_expression用于数学表达式或任何公式.以下是一个简单的示例,显示SQL数字表达式的使用 :
SQL> SELECT (15 + 6) AS ADDITION +----------+ | ADDITION | +----------+ | 21 | +----------+ 1 row in set (0.00 sec)
有几个已建成-in函数如avg(),sum(),count()等,以执行对表或特定表列的聚合数据计算.
SQL> SELECT COUNT(*) AS "RECORDS" FROM CUSTOMERS; +---------+ | RECORDS | +---------+ | 7 | +---------+ 1 row in set (0.00 sec)
日期表达式返回当前系统日期和时间值 :
SQL> SELECT CURRENT_TIMESTAMP; +---------------------+ | Current_Timestamp | +---------------------+ | 2009-11-12 06:40:23 | +---------------------+ 1 row in set (0.00 sec)
另一个日期表达式如下所示 :
SQL> SELECT GETDATE();; +-------------------------+ | GETDATE | +-------------------------+ | 2009-10-22 12:07:18.140 | +-------------------------+ 1 row in set (0.00 sec)