关系代数简介

\sigma_{condition}(TableName)

\sigma_{Price > 100}(Products)

1-- Selects all columns for products where the price is greater than 100
2SELECT *
3FROM Products
4WHERE Price > 100;

\pi_{Column1, Column2, ...}{(TableName)}

\pi_{CustomerName, Email}{(Customers)}

1-- Selects only the name and email columns from the Customers table
2SELECT CustomerName, Email
3FROM Customers;

\sigma_{Stock > 100}{(Products)}

\pi_{ProductName, Price}(...)

\pi_{ProductName, Price}(\sigma_{Stock > 100}{(Products)})

1-- Selects the name and price for products with more than 100 items in stock
2SELECT ProductName, Price
3FROM Products
4WHERE Stock > 100;

\rho_{NewName}{(TableName)}

\rho_{NewColumn1 \leftarrow OldColumn1, ...}{(TableName)}

\rho_{Customers}{(Clients)}

\rho_{Item \leftarrow ProductName,, Cost \leftarrow Price}\bigl(\pi_{ProductName,, Price}(Products)\bigr)

1-- Selects product names and prices, renaming the columns in the output
2SELECT
3    ProductName AS Item,
4    Price AS Cost
5FROM Products;