Learn in the textbookBasics of Data Operations (Insert/Update and the Relational Operations for Retrieval)
DefinitionProjection (射影) is the relational operation that keeps only the columns you need from a table to form a new table.
ExplanationProjection puts no condition on the rows and takes the values of the specified columns from every row. In SQL, the column names listed right after SELECT play the role of projection.
AnalogyLike cutting just the name and total-score columns out of a printed grade sheet and keeping them.
Memory hookRelational algebra writes it as π (pi), the Greek letter that matches the p of projection.