Why are they called left and right?

The terms "left" and "right" are often used in the context of data merging (join) or binary trees to denote two different data sets or branches. They are called so because, when visualizing or recording operations, one data set is positioned on the left, and the other on the right. This helps to clearly indicate which part is on which side, especially when describing algorithms or data operations. For example, in SQL join, LEFT JOIN means that all records from the left (first) table are retained, and matching records from the right are added.