查询员工表中所有员工的数量，正确的SQL语句是？
SELECT TOTAL(*) FROM employees
SELECT SUM(*) FROM employees
SELECT COUNT(*) FROM employees
SELECT AVG(*) FROM employees
要计算工资表（salary）中部门ID不等于2的所有员工的平均工资，应使用？
SELECT AVG(salary) FROM salary WHERE dept_id != 2
SELECT AVG(salary) FROM salary HAVING dept_id != 2
SELECT SUM(salary)/COUNT(dept_id) FROM salary WHERE dept_id != 2
SELECT AVG(salary) FROM salary GROUP BY dept_id != 2
查询订单表（orders）中订单金额的最大值，正确的是？
SELECT MAXIMUM(amount) FROM orders
SELECT TOP 1 amount FROM orders ORDER BY amount DESC
SELECT MAX(amount) FROM orders
SELECT HIGHEST(amount) FROM orders
统计学生表（students）中年龄大于18岁的学生人数，应使用？
SELECT TOTAL(age) FROM students WHERE age > 18
SELECT COUNT(*) FROM students WHERE age > 18
SELECT SUM(age) FROM students HAVING age > 18
SELECT COUNT(age) FROM students GROUP BY age > 18
以下哪个SQL语句能正确计算产品表（products）中价格低于500的商品总库存？
SELECT TOTAL(stock) FROM products WHERE price < 500
SELECT SUM(stock) FROM products WHERE price < 500
SELECT COUNT(stock) FROM products GROUP BY price < 500
SELECT SUM(stock) FROM products HAVING price < 500
要找出销售表（sales）中销量（quantity）的最小值，错误的语句是？
SELECT MIN(quantity) FROM sales
SELECT quantity FROM sales ORDER BY quantity ASC LIMIT 1
SELECT SMALLEST(quantity) FROM sales
SELECT MIN(quantity) AS min_qty FROM sales
在员工表中，统计有邮箱地址（email字段非空）的员工数量，应使用？
SELECT COUNT(email) FROM employees
SELECT COUNT(*) FROM employees WHERE email IS NOT NULL
SELECT SUM(email) FROM employees
SELECT COUNT(*) FROM employees
计算部门表（departments）中部门ID为2的平均预算（budget），并排除预算为NULL的记录，正确的是？
SELECT AVG(budget) FROM departments WHERE dept_id = 2 AND budget != NULL
SELECT AVG(budget) FROM departments WHERE dept_id = 2 AND budget IS NOT NULL
SELECT AVG(budget) FROM departments GROUP BY dept_id = 2
SELECT AVG(budget) FROM departments HAVING dept_id = 2
同时查询商品表（goods）中价格（price）的最高值和平均值，正确的是？
SELECT MAX(price) or AVG(price) FROM goods
SELECT MAX(price) AND AVG(price) FROM goods
SELECT MAX(price) + AVG(price) FROM goods
SELECT MAX(price) AS max_price, AVG(price) AS avg_price FROM goods
统计客户表（customers）中不同城市的数量，应使用？
SELECT COUNT(city) FROM customers
SELECT COUNT(DISTINCT city) FROM customers
SELECT SUM(city) FROM customers
SELECT COUNT(ALL city) FROM customers