Sql how to compare strings?

Question

Yii

Hello Dear SQL gurus
Such a question, there is a table of loans (loans), it is necessary to implement such an opportunity so that the person who issued a loan to someone could not take a loan from the same person to whom he issued another loan (it’s really difficult for me to formulate this) .
You can issue loans as much as you like and take as much as you like, on the screen, I wanted to show that id 4 could not stand up in those fields where id 1 has already taken a loan from me. I just need a check. If it is not possible to solve this in pure Sql, then you can please tell me in yii2.

Answer 1

At least here you need to give an example of tables of people and loans, and their relationships. Then let's continue