For the document Control Assistant who was hired on 2009/1/22, what is the percentage of private documents did he/she have?
Document Control Assistant refers  to the  JobTitle = 'Document Control Assistant'; hired on 2009/1/22 means the person's hiring date is HireDate = '2009-01-22'; private documents indicate that DocumentSummary is null; DIVIDE(COUNT(DocumentSummary is null), COUNT(DocumentSummary))*100
SELECT CAST(SUM(CASE WHEN T1.DocumentSummary IS NOT NULL THEN 1 ELSE 0 END) AS REAL) / COUNT(T1.DocumentSummary) FROM Document AS T1 INNER JOIN Employee AS T2 ON T1.Owner = T2.BusinessEntityID WHERE T2.JobTitle = 'Document Control Assistant' AND T2.HireDate = '2009-01-22'