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!= (A leftjoin B on (Pab)) join C on (Pbc)
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SEMI joins work a little bit differently. A semijoin can be reassociated
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of the righthand side. Likewise, an inner join, left join, or antijoin
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ANTI joins work approximately like LEFT joins, except that identity 3
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fails if the join to C is an antijoin (even if Pbc is strict, and in