A database testing system may generate, based on a randomization process, test data for a database. The system may generate a query that, when processed by the database, produces a first result that comprises the row. The system may modify the query based on a randomization process and in view of the semantic properties of the query language associated with the queries. The system may predict the outcome of the modifies query based on the semantic properties and on the generated test data. The system may verify that the modified query, when processed by the database, produces results consistent with the prediction.