表达式是一个或多个值,运算符以及对一个值求值的SQL函数的组合,这些SQL EXPRESSION类似于公式,它们以查询语言编写。您还可以使用它们来查询数据库以获取一组特定的数据。
考虑一下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,如下所述-
现在让无涯教程详细讨论每个。
SQL布尔表达式基于匹配单个值来获取数据。以下是语法
链接:https://www.learnfk.comhttps://www.learnfk.com/sql/sql-expressions.html
来源:LearnFk无涯教程网
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 | Learnfk | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+ 7 rows in set (0.00 sec)
下表是一个简单示例,显示了各种SQL布尔表达式的用法-
SQL> SELECT * FROM CUSTOMERS WHERE SALARY = 10000; +----+-------+-----+---------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+-------+-----+---------+----------+ | 7 | Learnfk | 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] ;
此处,numerical_expression 用于数学表达式或任何公式,以下是显示SQL数值表达式用法的简单示例-
SQL> SELECT (15 + 6) AS ADDITION +----------+ | ADDITION | +----------+ | 21 | +----------+ 1 row in set (0.00 sec)
有几个内置函数,如avg(),sum(),count()等,用于对表或特定表列执行所谓的聚合数据计算。
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 | +---------------------+ | 2019-11-12 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 | +-------------------------+ | 2019-10-22 12:07:18.140 | +-------------------------+ 1 row in set (0.00 sec)
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