SQL programming task02 job - basic query and sorting

1 Operator

1.1 Arithmetic operator

meaning	Operator
Add	+
Subtraction	-
Multiplication	*
division	/

1.2 Comparison operator

Operator	meaning
=	and ~ equal
<>	and ~ It's not equal
>=	Greater than or equal to
>	Greater than
<=	Less than or equal to
<	Less than

2 Exercise part 1 - Inquire about

2.1 To write product form

SQL The code is as follows ：

CREATE TABLE product
(product_id    CHAR(4)      NOT NULL,
product_name   VARCHAR(100) NOT NULL,
product_type   VARCHAR(32)  NOT NULL,
sale_price     INTEGER      DEFAULT 0,
purchase_price INTEGER ,
regist_date    DATE ,
PRIMARY KEY (product_id)); 

INSERT INTO product VALUES('0001', 'T shirt ', ' clothes ', 1000, 500, '2009-09-20');
INSERT INTO product VALUES('0002', ' Punch ', ' Office Supplies ', 500, 320, '2009-09-11');
INSERT INTO product VALUES('0003', ' motion T T-shirt ', ' clothes ', 4000, 2800, NULL);
INSERT INTO product VALUES('0004', ' kitchen knife ', ' Kitchenware ', 3000, 2800, '2009-09-20');
INSERT INTO product VALUES('0005', ' pressure cooker ', ' Kitchenware ', 6800, 5000, '2009-01-15');
INSERT INTO product VALUES('0006', ' Fork ', ' Kitchenware ', 500, NULL, '2009-09-20');
INSERT INTO product VALUES('0007', ' Clean the board ', ' Kitchenware ', 880, 790, '2008-04-28');
INSERT INTO product VALUES('0008', ' Ball pen ', ' Office Supplies ', 100, NULL, '2009-11-11');

give the result as follows ：

2.2 Query statement

Write a SQL sentence , from product（ goods ） Select from the table “ Registration date （regist stay 2009 year 4 month 28 After the day ” The goods , Query results should contain product name and regist_date Two .
SQL Code ：

select product_name,regist_date
 from product
where 	not regist_date = '2009-01-15'
    and not regist_date = '2008-04-28';

Query results ：

2.3 NULL Several incorrect query formats

Please say yes to product The table is executed as follows 3 strip SELECT The return result of statement

SELECT *
  FROM product
 WHERE purchase_price = NULL;

SELECT *
  FROM product
 WHERE purchase_price <> NULL;

SELECT *
  FROM product
 WHERE product_name > NULL;

No record can be obtained from the above three query results , give the result as follows ：

reason ：
selection NULL When recording , You need to use... In conditional expressions IS NULL Operator . I hope the choice is not NULL When recording , You need to use... In conditional expressions IS NOT NULL Operator .

2.4 Conditional query statements + Operator

SELECT Statements can be made from product Take out from the table “ Unit sales price （saleprice） Compared with the purchase price （purchase price） Higher than 500 Above JPY ” The goods . Please write two items that can get the same result SELECT sentence . The results are shown below .

SQL The statement is as follows ：

SELECT product_name, sale_price, purchase_price
  FROM product
 WHERE sale_price-purchase_price >= 500;
 
SELECT product_name, sale_price, purchase_price
  FROM product
 WHERE sale_price-500 >= purchase_price;

SELECT product_name, sale_price, purchase_price
  FROM product
 WHERE sale_price >= purchase_price +500;

2.5 Conditions of the query + The query result is set to another column

SQL Code ：

SELECT product_name, product_type,sale_price*0.9-purchase_price as profit
  FROM product
 WHERE sale_price*0.9-purchase_price>100
  and (product_type=' Office Supplies ' or product_type=' Kitchenware ');

give the result as follows ：

3 polymerization & Sort

3.1 Aggregate functions

COUNT： Count the number of records in the table （ Row number ）
SUM： Calculate the total value of the data in the numerical column of the table
AVG： Calculate the average value of the data in the numerical column in the table
MAX： Find the maximum value of the data in any column of the table
MIN： Find the minimum value of data in any column in the table

3.2 WHERE Under the condition of GROUP BY sentence

SELECT purchase_price, COUNT(*)
  FROM product
 WHERE product_type = ' clothes '
 GROUP BY purchase_price;

GROUP BY There are strict requirements for the writing order of clauses , Failure to comply with the requirements will result in SQL Unable to execute normally , The order of clauses that have appeared at present is ：
1 SELECT → 2. FROM → 3. WHERE → 4. GROUP BY
The first three items are used to filter the data ,GROUP BY Process the filtered data

3.3 use HAVING Get specific groups

--  Numbers 
SELECT product_type, COUNT(*)
  FROM product
 GROUP BY product_type
HAVING COUNT(*) = 2;

3.4 ORDER BY Sort

--  Descending order 
SELECT product_id, product_name, sale_price, purchase_price
  FROM product
 ORDER BY sale_price DESC;
--  Multiple sort keys 
SELECT product_id, product_name, sale_price, purchase_price
  FROM product
 ORDER BY sale_price, product_id;
--  When the column name used for sorting contains NULL when ,NULL It will be summarized at the beginning or end .
SELECT product_id, product_name, sale_price, purchase_price
  FROM product
 ORDER BY purchase_price;

problem ： GROUP BY Cannot use... In clause SELECT Clause , But in ORDER BY You can use aliases in clauses . Why is it GROUP BY Not in... But in ORDER BY It's ok ？
This is because SQL In the use of HAVING When clause SELECT The order of statements is ：
FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY
among SELECT The order of execution is GROUP BY After Clause ,ORDER BY Before clause . in other words , When in ORDER
BY When using aliases in , I already know SELECT The alias set does not exist , But in GROUP BY When you use aliases in, you don't know the existence of aliases , So it can't be ORDER
BY You can use aliases in , But in GROUP BY Alias cannot be used in

4 The second part of the exercise - polymerization & Sort

4.1 Please point out the following SELECT All the grammatical errors in the statement .

SELECT product_id, SUM（product_name）
-- Ben SELECT There is an error in the statement .
  FROM product 
 GROUP BY product_type 
 WHERE regist_date > '2009-09-01';

Explain ：

• SUM, Calculate the total value of the data in the numerical column of the table ,product_name Column is not numeric column data .
• Punctuation marks, such as parentheses, commas, semicolons, use full angles , Half width should be used .
• GROUP BY And WHERE The order is reversed , And now WHERE A semicolon is not required after the statement .
• SELECT If the column name appears in the clause , Can only be GROUP BY The column name specified in clause （ That's the aggregate bond ）.

4.2 HAVING Filter aggregate groups

Please write one SELECT sentence , Find out the sales unit price （ sale_price Column ） The total value is greater than the purchase unit price （ purchase_price Column ） Total value 1.5 Times the commodity category . The results are shown below .

SQL Code ：

SELECT product_type,sum(sale_price),sum(purchase_price)
  FROM product 
 GROUP BY product_type
 HAVING sum(sale_price) > 1.5*sum(purchase_price);

4.3 ORDER BY Sort

We used it before SELECT The sentence is selected product（ goods ） All the records in the table . We used ORDERBY Clause to specify the sort order , But now I can't remember how it was designated . Please refer to the following execution results , reflection ORDERBY The content of the clause .

SQL Code ：

SELECT *
  FROM product
 ORDER BY if(isnull(regist_date),0,1),regist_date DESC, sale_price;

Reference resources ：DataWhale SQL Team learning