What are the document dates of all documents with the document type code "BK"?
SELECT T2.Document_Date FROM Projects AS T1 JOIN Documents AS T2 ON T1.project_id   =   T2.project_id WHERE T2.Document_Type_Code  =   "BK"
document dates of all documents = SELECT Document_Date; with the document type code \"BK\" = WHERE Document_Type_Code = \"BK\"; joining tables Projects and Documents on project_id = JOIN ... ON T1.project_id = T2.project_id;