What is the difference between an empty string and a NULL value?

Empty strings ( "") and NULLvalues ​​are two different ways of representing missing or undefined data in a database with the following differences:

1. Meaning:

   • Empty String ( ""): Represents a valid, existing string, but its content is empty.
   • NULLValue: Indicates a missing value or undefined data. It means that the field is not assigned any value.

2. Storage method:

   • Empty string ( ""): As a specific string value, it occupies storage space.
   • NULLValue: Indicates a missing value and does not occupy additional storage space.

3. Query behavior:

   • Empty String ( ""): Can be compared using equal ( =) or not equal ( <>) operators, for string fields, it will match records with empty string.
   • NULLValue: needs to be compared using the IS NULLor IS NOT NULLoperator, which represents a missing value, unlike an empty string.

In database design, choose an appropriate way to represent missing or undefined data according to your needs. Typically, if a field is optional and allowed to be empty, you can use an empty string ( "") as the default value. Values ​​can be used if the field is required and missing values ​​need to be explicitly represented NULL.