SQL NULL 是用于表示缺失值,表中的NULL值是显示为空白的字段中的值。
NULL - 语法
创建表时 NULL 的基本语法。
SQL> CREATE TABLE CUSTOMERS( ID INT NOT NULL, NAME VARCHAR (20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR (25) , SALARY DECIMAL (18, 2), PRIMARY KEY (ID) );
NULL - 示例
选择数据时,您必须使用 IS NULL 或 IS NOT NULL 运算符来检查NULL值。
考虑以下具有记录的CUSTOMERS表,如下所示。
+----+----------+-----+-----------+----------+ | 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 | | | 7 | Learnfk | 24 | Indore | | +----+----------+-----+-----------+----------+
现在,下面是 IS NOT NULL 运算符的用法。
SQL> SELECT ID, NAME, AGE, ADDRESS, SALARY FROM CUSTOMERS WHERE SALARY IS NOT NULL;
这将产生以下输出-
+----+----------+-----+-----------+----------+ | 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 | +----+----------+-----+-----------+----------+
现在,下面是 IS NULL 运算符的用法。
SQL> SELECT ID, NAME, AGE, ADDRESS, SALARY FROM CUSTOMERS WHERE SALARY IS NULL;
这将产生以下输出-
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 6 | Komal | 22 | MP | | | 7 | Learnfk | 24 | Indore | | +----+----------+-----+-----------+----------+