SQL NULL 是用于表示缺失值的术语.表中的NULL值是字段中显示为空白的值.
具有NULL值的字段是没有值的字段.理解NULL值不是零值或包含空格的字段非常重要.
基本语法创建表时 NULL .
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) );
这里, NOT NULL 表示该列应始终接受给定数据类型的显式值.有两列我们没有使用NOT NULL,这意味着这些列可能是NULL.
具有NULL值的字段是在创建记录期间留空的字段.
选择数据时,NULL值可能会导致问题.但是,因为在将未知值与任何其他值进行比较时,结果始终未知且未包含在结果中.您必须使用 IS NULL 或 IS NOT NULL 运算符来检查NULL值.
考虑以下CUSTOMERS表,记录如下所示.
+----+----------+-----+-----------+----------+ | 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 | Muffy | 24 | Indore | | +----+----------+-----+-----------+----------+
现在,以下是使用 IS NOT NULL 运算符.
SQL> SELECT ID, NAME, AGE, ADDRESS, SALARY FROM CUSTOMERS WHERE SALARY IS NOT NULL;
这会产生以下结果 :
+----+----------+-----+-----------+----------+ | 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,以下是使用 IS NULL 运算符.
SQL> SELECT ID, NAME, AGE, ADDRESS, SALARY FROM CUSTOMERS WHERE SALARY IS NULL;
这会产生以下结果 :
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 6 | Komal | 22 | MP | | | 7 | Muffy | 24 | Indore | | +----+----------+-----+-----------+----------+