Abstract A study of 13–14 year‐old students' performance on two parallel logical reasoning tasks related to database searches is reported. In one task, given a logical sentence defining a search, students had to construct the equivalent data structure. In the other, given a data structure, they had to construct the equivalent logical sentence. Binary logical sentences including AND, OR and NOT were investigated, including tautology and contradiction. Students were more successful with sentences using AND than with sentences using OR. Negation generally produced difficulties, and essentially none could manage tautology and contradiction. Sentences using OR were more difficult, especially when they involved NOT. A common pattern was to treat OR as if it were AND. The two tasks, though formally parallel, were very different in difficulty, with the construction of sentences from a data structure being the more difficult. Both tasks were administered before and after training in the use of a database. A negligible difference in performance was