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1. Contains a large number of interview question banks
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3. The questions of brushing questions are in accordance with the process from simple to difficult
1. Grouping
Summary: Grouping is established in the GROUP BY clause of the SELECT statement. Grouping allows data to be divided into logical groups so that aggregate calculations can be performed on each group
Main function:
| ORDER BY | GROUP BY |
|---|---|
| Sorting produces output | Group rows. but the output may not be in the order of the groups |
| Any column can be used (even non-selected columns can be used) | Only select columns or expression columns are possible, and each select column expression must be used |
| not necessarily required | If you use a column (or expression) with an aggregate function, you must use |
2. Brush questions
1. Brush Question 1
(1) Topics
Question: Return the number of rows for each order number
Description : Write an SQL statement to return the number of rows (order_lines) of each order number (order_num), and sort the results in ascending order by order_lines.
(2) Difficulty level
Difficulty Level: Beginner
(3) Example
输入：
DROP TABLE IF EXISTS `OrderItems`;
CREATE TABLE IF NOT EXISTS `OrderItems`(
order_num VARCHAR(255) NOT NULL COMMENT '商品订单号'
);
INSERT `OrderItems` VALUES ('a002'),('a002'),('a002'),('a004'),('a007');
输出：
a004|1
a007|1
a002|3(4) Code
select order_num,count(*) as order_lines
from OrderItems
group by order_num
order by order_lines(5) Running screenshot
2. Brush question 2
(1) Topics
Topic: Lowest cost product per supplier
Description: Write an SQL statement that returns a field named cheapest_item that contains the lowest cost product per supplier (using prod_price from the Products table), and then sort the results in ascending order from lowest cost to highest cost.
(2) Difficulty level
Difficulty level: medium
(3) Example
输入：
DROP TABLE IF EXISTS `Products`;
CREATE TABLE IF NOT EXISTS `Products` (
`vend_id` VARCHAR(255) NOT NULL COMMENT '供应商ID',
`prod_price` DOUBLE NOT NULL COMMENT '产品价格'
);
INSERT INTO `Products` VALUES ('a0011',100),
('a0019',0.1),
('b0019',1000),
('b0019',6980),
('b0019',20);
输出：
vend_id|cheapest_item
a0019|0.100
b0019|20.000
a0011|100.000(4) Code
select vend_id,prod_price as cheapest_item
from (
select row_number() over(partition by vend_id order by prod_price asc) as row_num,
vend_id,prod_price
from Products) temp
where row_num = 1
order by cheapest_item; (5) Running screenshot
3. Brush question three
(1) Topics
Question: Return the order number of all orders whose total order quantity is not less than 100
Description: Please write a SQL statement to return all order numbers whose total order quantity is not less than 100. The final result is sorted in ascending order by order number.
(2) Difficulty level
Difficulty level: medium
(3) Example
输入：
DROP TABLE IF EXISTS `OrderItems`;
CREATE TABLE IF NOT EXISTS `OrderItems`(
order_num VARCHAR(255) NOT NULL COMMENT '商品订单号',
quantity INT(255) NOT NULL COMMENT '商品数量'
);
INSERT `OrderItems` VALUES ('a1',105),('a2',200),('a4',1121),('a5',10),('a7',5);
输出：
a1
a2
a4(4) Code
SELECT order_num
FROM OrderItems
GROUP BY order_num
HAVING SUM(quantity) >= 100
ORDER BY order_num(5) Running screenshot
4. Brush Question 4
(1) Topics
Topic: Calculate the sum
Description: Write a SQL statement, aggregate according to the order number, return all order numbers whose total order price is not less than 1000, and sort the final result in ascending order by order number. Hint: total price = item_price multiplied by quantity
(2) Difficulty level
Difficulty level: medium
(3) Example
输入：
DROP TABLE IF EXISTS `OrderItems`;
CREATE TABLE IF NOT EXISTS `OrderItems`(
order_num VARCHAR(255) NOT NULL COMMENT '商品订单号',
item_price INT(16) NOT NULL COMMENT '售出价格',
quantity INT(16) NOT NULL COMMENT '商品数量'
);
INSERT `OrderItems` VALUES ('a1',10,105),('a2',1,1100),('a2',1,200),('a4',2,1121),('a5',5,10),('a2',1,19),('a7',7,5);
输出：
a1|1050.000
a2|1319.000
a4|2242.000(4) Code
select order_num,sum(item_price*quantity) total_price from OrderItems
group by order_num
having total_price>=1000
order by order_num(5) Running screenshot
5. Brush Question 5
(1) Topics
Subject: Error Correction 3
Description : Execute after modifying the following code correctly
SELECT order_num, COUNT(*) AS items
FROM OrderItems
GROUP BY items
HAVING COUNT(*) >= 3
ORDER BY items, order_num;
(2) Difficulty level
Difficulty level: easy
(3) Example
输入：
DROP TABLE IF EXISTS `OrderItems`;
CREATE TABLE IF NOT EXISTS `OrderItems`(
order_num VARCHAR(255) NOT NULL COMMENT '商品订单号'
);
INSERT `OrderItems` VALUES ('a002'),('a002'),('a002'),('a004'),('a007');
输出：
a002|3(4) Code
SELECT order_num, COUNT(*) AS items
FROM OrderItems
GROUP BY order_num
HAVING items >= 3
ORDER BY items, order_num;(5) Running screenshot
3. Summary
This article is the thirteenth day of a series of MySQL essay questions!
Welcome friends to brush the questions together, click on the link on the right ------ brush questions network