A system for optimizing SQL queries in a relational database management system using magic decorrelation. After representing an assembly of multiple SQL queries in computer memory using data structures configured in the query box representation ("QBR") format, queries amenable to decorrelation are identified, and FEED and ABSORB stages are applied to rewrite the QBR in decorrelated form. The FEED stage generates a set of correlation values that the subquery can use to decorrelate. The ABSORB stage decorrelates the subquery using the correlation values prepared by the FEED stage, resulting in the correlation bindings being absorbed into the subquery. The invention thereby takes full advantage of set-at-a-time, rather than tuple-at-a-time, processing.