Learn in the textbookBasics of Data Operations (Insert/Update and the Relational Operations for Retrieval)
DefinitionA join (結合) is a relational operation that links two or more tables by pairing rows whose values in a common column (共通の列) are equal, combining them into one table.
ExplanationThe link is usually a pair made of one table's primary key (主キー) and the foreign key (外部キー) in the other table that holds those values. Because the result table carries the columns of both tables, items that exist in only one of them can be handled together.
AnalogyLike checking the product number on an order slip against the same number in a catalog and writing the product name beside it.
Memory hookRelational algebra writes it as ⋈, a bow-tie shape that looks like the left and right tables tied together in the middle.