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create table t1 (a int, b int not null, c int null, d int not null);
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insert into t1 values (0,0, 0, 0);
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insert into t1 values (NULL, NULL, NULL, NULL);
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ERROR 23000: Column 'b' cannot be null
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insert into t1 values (NULL, 1, NULL, 1);
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insert into t1 values (1,2,3,NULL);
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ERROR 23000: Column 'd' cannot be null