(Cambridge)ACM Sigact-Sigmed Symposium on Principles of Database Systems ACM Sigact-Sigmed Symposium on Principles of Database Systems ACM Sigact-Sigmod Symposium on Principles of Database Systems Fourth ACM SIGACT-SIGMOD Symposium on Principles of Database Systems ACM SIGACT-SIGMOD Symposium on Principles of Database Systems. (Portland)Fourth ACM SIGACT-SIGMOD Symposium on Principles of Database Systems ACM Sigact-Sigmod Symposium on Principles of Database Systems: Proceedings