SQL (Structured Query Language) is the industry standard for querying databases. Unfortunately, users commit many errors when using SQL. To understand the causes of user errors, the author analyzed the task of query writing while considering the characteristics of human cognition. This analysis revealed multiple cognitive causes of a frequent and troublesome error, join clause omission. This error not only wastes users' time for error correction, but it also returns answers from the database that may be undetectably wrong. The cognitive causes of this onerous error were tested experimentally, and that experiment revealed that the four possible causes of join clause omission all contributed to the error. Specifically, the frequency of this error increased because 1)some people never learned the procedure, 2)an explicit clue to write the “join clause” was absent from the problem statement, 3)the load on working memory caused by writing intervening clauses made the users forget to include the “join clause,” and 4)users inappropriately reused the procedure appropriate for a single table query (which requires no join clause) when a “join clause” is indeed necessary.