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create table t1 (pk int primary key, col_int int, col_int_unique int unique key);
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insert into t1 values (1,NULL,2), (2,0,0);
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create table t2 (pk int primary key, col_int int, col_int_unique int unique key);
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insert into t2 values (1,0,1), (2,0,2);
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select * from t1 left join t2
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on t1.col_int_unique = t2.col_int_unique and t1.col_int = t2.col_int