In a venn diagram the sum of the area combined by A and B can never be larger than A+B.
In a join the result can multiply and you get up to A*B rows instead.
Hence the geometrical representation can mislead you into thinking your data is smaller than what it actually is. As a quick mental model i think the diagrams can help but you have to understand they are only a reminder of how the join is being matched, not a representation of the complete data set.
In a join the result can multiply and you get up to A*B rows instead.
Hence the geometrical representation can mislead you into thinking your data is smaller than what it actually is. As a quick mental model i think the diagrams can help but you have to understand they are only a reminder of how the join is being matched, not a representation of the complete data set.