Hard deduction SQL statement exercises, wrong question records

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This blog is a record of Li Kou SQL Wrong questions in exercises .（ Original title transmission gate ）

Catalog

1. Looking for user references  

2. Calculate special bonuses

3. Delete duplicate messages

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1. Looking for user references  

Given table  customer , It contains all the customer information and their references .

+------+------+-----------+
| id   | name | referee_id|
+------+------+-----------+
|    1 | Will |      NULL |
|    2 | Jane |      NULL |
|    3 | Alex |         2 |
|    4 | Bill |      NULL |
|    5 | Zack |         1 |
|    6 | Mark |         2 |
+------+------+-----------+

Write a query statement , Return a list of customers , The number of the customer's recommender in the list is   No  2.

For the sample data above , The result is ：

+------+
| name |
+------+
| Will |
| Jane |
| Bill |
| Zack |
+------+

SQL sentence ：

 IFNULL(expression_1,expression_2);

If expression_1 by NULL, be IFNULL The function returns expression_2; Otherwise return to expression_1 Result .

select name from customer where ifnull(referee_id,0)!=2

2. Calculate special bonuses

Write a SQL Query statement , Calculate the bonus for each employee . If an employee's id It's odd and his name doesn't start with 'M' start , Then his bonus is his salary 100%, Otherwise, the bonus is 0. For the returned result set, please follow employee_id Sort .

The query result format is shown in the following example .

 Input ：
Employees  surface :
+-------------+---------+--------+
| employee_id | name    | salary |
+-------------+---------+--------+
| 2           | Meir    | 3000   |
| 3           | Michael | 3800   |
| 7           | Addilyn | 7400   |
| 8           | Juan    | 6100   |
| 9           | Kannon  | 7700   |
+-------------+---------+--------+
 Output ：
+-------------+-------+
| employee_id | bonus |
+-------------+-------+
| 2           | 0     |
| 3           | 0     |
| 7           | 7400  |
| 8           | 0     |
| 9           | 7700  |
+-------------+-------+
 explain ：
 Because employees id It's even , So employees id  yes 2 and 8 The bonus for two of our employees is 0.
 Employee id by 3 Because his name is 'M' start , therefore , Bonus is 0.
 The other employees got a 100% bonus .
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SQL sentence ：

Mod(a,b) :a / b The remainder of ;         

LEFT( Field name , Subscript )： return ' Field name ' From the left ' Subscript ' Characters .

select employee_id,if(mod(employee_id,2)!=0 
and left(name,1)!='M',salary,0) bonus 
from employees order by employee_id

3. Delete duplicate messages

Write a SQL Delete statements to Delete All duplicate emails , Keep only one id The smallest and only email . With In any order Return result table . The query result format is as follows .

 Input : 
Person  surface :
+----+------------------+
| id | email            |
+----+------------------+
| 1  | [email protected] |
| 2  | [email protected]  |
| 3  | [email protected] |
+----+------------------+
 Output : 
+----+------------------+
| id | email            |
+----+------------------+
| 1  | [email protected] |
| 2  | [email protected]  |
+----+------------------+
 explain : [email protected] Repeat twice . We keep the smallest Id = 1.

SQL sentence ：

DELETE from Person Where Id not in (
    Select Id From(Select MIN(Id) as id From Person Group by Email) t)

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