According to the Jennifer summarizing relational algebra. I think today's video is made of the select and project operators and union difference and intersection, therenaming operator, and different notations for expressions of relational algebra. In the first part, she introduced all kinds of select and project operators in various types of joins. If you want to use the select operator to select certain rows you want, the select operator is denoted by a Sigma with a subscript--that's the condition that's used to filter the rows that we extract from the relations. σGPA > 3.7 it means that you use the operator to indicate that you want to screen students with a GPA of over 3.7. And about the project operator is written using the Greek pi symbol, and now the subscript is a list of the column names that we would like to extract.
In the second part, she told some operators. The first operator is the union operator, it merges the information you need from two tables into a new table, as long as the two schemas are consistent. The second operator is difference operator, it means that if we want to find students who have not applied to college, we can get the ID of the student who has not applied by subtracting the ID of the student who has applied to college from all the student IDs. Our last operator is the rename operator. “The rename operator is necessary to express certain queries in relational algebra. The rename operator uses the Greek symbol rho.” The rename operator simply means that two attributes can be renamed only if they are identical. And the second use of the rename operator is a little more complicated and quite a bit more important actually which is disambiguation in self.