Both functions take two arguments. With PATINDEX, you must include percent signs before and after the pattern, unless you are looking for the pattern as the first (omit the first %) or last (omit the last %) characters in a column. For CHARINDEX, the pattern cannot include wildcard characters. The second argument is a character expression, usually a column name, in which Adaptive Server searches for the specified pattern.
Example of CHARINDEX:
USE AdventureWorks;
GO
SELECT CHARINDEX(‘ensure’, DocumentSummary)
FROM Production.Document
WHERE DocumentID = 3;
GO
Examples of PATINDEX:
USE AdventureWorks;
GO
SELECT PATINDEX(‘%ensure%’,DocumentSummary)
FROM Production.Document
WHERE DocumentID = 3;
GO
Source