单项选择题
A SELECT statement can be used to perform these three functions: 1. Choose rows from a table. 2. Choose columns from a table 3. Bring together data that is stored in different tables by creating a link between them. Which set of keywords describes these capabilities? （）
A. difference, projection, join B. selection, projection, join C. selection, intersection, join D. intersection, projection, join E. difference, projection, product
单项选择题 Which is a valid CREATE TABLE statement? （）
多项选择题 Which three are true? （）
多项选择题 Which two statements are true about constraints? （）
