SQL handles null values

Note: All underscore + italic statements are optional statements

SQL statement:

Select…  From…  Where 列名 is not null…

Note that "=null" is not allowed

Summary of null value handling in current DBMS:

• A null value does not satisfy any lookup criteria except is not null
• If null participates in an arithmetic operation, the value of the arithmetic expression is null
• If null participates in the comparison operation, the result can be regarded as 0 (can be regarded as unknown in SQL-92)
• If null participates in the aggregation operation, other aggregation functions except count(*) ignore null

Example ①: Find the names of students whose age value is empty

Select Sname From Student
Where Sage is null;

Example ②: the following table:

Select AVG(Score) From SC    → the result is (92+55)/2 = 73.5

Select COUNT(*) From SC     → result is 3