'column1' 'column2' 'column3' is different than 'column1,column2,column3'
So, you must compare the whole string:
IF (col_names = 'column1,column2,column3')
If you want to compare each variable separately, then you must pass them separate in your procedure call:
This warning is shown for any other but the supported MySQL versions (at the time being these are 5.6, 5.7 and 8.0). The application uses language features and other resources that may or may not work with other servers. It's actually not tested, which is the main reason why this warning comes up.
MariaDB might be based on MySQL, but over time it more and ...
IN ( SELECT ... ) is often an inefficient way to design a query. There are two alternatives below. Also, your IN was asking to fetch the entire array; this is probably causing terrible performance. You only need to check for the "existence" of one customer.
Note that each formulation checks for one "customer" in Table B and does not fetch the entire ...