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As a result, the evaluation of SQL queries on databases with nulls may produce answers that are just plain wrong. However, SQL evaluation can be modified, at least for relational algebra queries, to approximate certain answers, i.e., return only correct answers. We examine recently proposed approximation schemes for certain answers and analyze their complexity, both theoretical bounds and real-life behavior<\/jats:p>","DOI":"10.1145\/3156655.3156657","type":"journal-article","created":{"date-parts":[[2017,11,1]],"date-time":"2017-11-01T12:45:06Z","timestamp":1509540306000},"page":"5-16","update-policy":"https:\/\/doi.org\/10.1145\/crossmark-policy","source":"Crossref","is-referenced-by-count":14,"title":["Correctness of SQL Queries on Databases with Nulls"],"prefix":"10.1145","volume":"46","author":[{"given":"Paolo","family":"Guagliardo","sequence":"first","affiliation":[{"name":"The University of Edinburgh"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Leonid","family":"Libkin","sequence":"additional","affiliation":[{"name":"The University of Edinburgh"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"320","published-online":{"date-parts":[[2017,10,31]]},"reference":[{"key":"e_1_2_1_1_1","volume-title":"Foundations of Databases","author":"Abiteboul S.","year":"1995"},{"key":"e_1_2_1_2_1","doi-asserted-by":"publisher","DOI":"10.1016\/0304-3975(51)90007-2"},{"key":"e_1_2_1_3_1","doi-asserted-by":"crossref","DOI":"10.1017\/CBO9781139060158","volume-title":"Foundations of Data Exchange","author":"Arenas M.","year":"2014"},{"key":"e_1_2_1_4_1","volume-title":"Morgan Kaufmann","author":"Celko J.","year":"1995"},{"key":"e_1_2_1_5_1","doi-asserted-by":"publisher","DOI":"10.1109\/69.842265"},{"key":"e_1_2_1_6_1","volume-title":"Relational Database Writings 1991-1994.","author":"Codd E. 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