Can I use… nullif(…, …)

SQL nullif returns the value of the first argument unless it equals the second argument—then it returns the null value.

SELECT NULLIF(string_value, 'unknown')
  FROM …

The example returns the value of string_value unless it contains the the text unknown—then it returns the SQL null value.0

Related

• Tutorials: Null — Absent Data, Case — Conditional Expressions

• Other forms: case … when … (simple case), case when … (searched case).

Normative References

The nullif expression is defined in ISO/IEC 9075:2016-2 as mandatory feature F261-03.