mysql - How to process and integrate the contents of two tables into a third table in php

The database has three tables A B C. There are millions of data in table A and there are fields keys in them. This field is used for replacement. Tables B and C also have keys fields. Now I want to randomly select three keys from the two tables B and C and combine them one by one to replace them. I want to ask an expert to give me some advice on removing the keys field in table A. It would be great if there is some ready-made code that I can learn from. Thank you

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The database has three tables A B C. There are millions of data in table A and there are fields keys in them. This field is used for replacement. Tables B and C also have keys fields. Now I want to randomly select three keys from the two tables B and C and combine them one by one to replace them. I want to ask an expert to give me some advice on removing the keys field in table A. It would be great if there is some ready-made code that I can learn from. Thank you

You already know the processing logic, you just need to turn it into code.
The most you can do is add some if else logical judgments. So quickly write a php to implement it yourself