- Relational DB table are built on a strong mathematical function called relational theory.
- Relations, Tables or operated on formal language called relational algebra.
- SQL is a commercial interpretation of the relational algebra.
- Three concepts from relational theory encompass the capability of SELECT statement.
- Projection
- Selection
- Joining
Projection: It chooses a column from a table that is returned by a query. We can choose as many of column we needed. Eg: select three columns Stud_Id, Stud_Name & Stud_Mark;
Selection: It chooses a row in a table that return by a query. Various criteria can be used to restrict the rows that are restricted. Eg: select Stud_Id who have ID = 30;
Joining: It used to join the data which are stored in different tables. SQL joins used to display those data’s together. Eg: joining two tables & retrieve information.
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