Want to do a database data comparison tool, what ideas to achieve the most efficient?

A database a normal update data, want to synchronize data from another database B to database A, that is, database A is ultimately to include data in database B, the daily data comparison, consult the implementation of ideas.
If you use nested loops for data comparisons, the efficiency is too low and becomes slower as the amount of data increases.

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A database a normal update data, want to synchronize data from another database B to database A, that is, database A is ultimately to include data in database B, the daily data comparison, consult the implementation of ideas.
If you use nested loops for data comparisons, the efficiency is too low and becomes slower as the amount of data increases.

It is suggested to look at the structure of the Navicat, data synchronization tools, there should be similar to open source projects, writing this tool, must be efficient, comprehensive to consider

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