单项选择题
A. You can join a maximum of two tables through an equijoin.
B. You can join a maximum of two columns through an equijoin.
C. You specify an equijoin condition in the SELECT or FROM clauses of a SELECT statement.
D. To join two tables through an equijoin, the columns in the join condition must be primary key and foreign key columns.
E. You can join n tables (all having single column primary keys) in a SQL statement by specifying a minimum of n-1 join conditions.
单项选择题 You need to calculate the total of all salaries in the accounting department. Which group function should you use? （）
单项选择题 You define a multiple-row subquery in the WHERE clause of an SQL query with a comparison operator "=". What happens when the main query is executed? （）
多项选择题 Which three statements correctly describe the functions and use of constraints? （）