MySQL - adding, deleting, querying and modifying tables

Add or delete check change CRUD:Create（ establish ）,Retrieve（ Read ）,Update（ to update ）,Delete（ Delete ）

One .Create

Create a table to hold student information

 1. Single line data + Insert all columns

 

 2.  Multi row data + Specify column insertion

 3. Insert or update

When primary key or unique key conflicts , You can select the syntax for synchronous update ：

INSERT INTO students (id, sn, name) VALUES (1, 1, 'zhang')
ON DUPLICATE KEY UPDATE sn = 1, name = 'zhang';
Query OK, 2 rows affected (0.47 sec)
-- 0 row affected:  There are conflicting data in the table , But the value of the conflicting data and  update  The values are equal 
-- 1 row affected:  There are no conflicting data in the table , Data is inserted 
-- 2 row affected:  There are conflicting data in the table , And the data has been updated 
--  adopt  MySQL  Function to get the number of affected data rows 
SELECT ROW_COUNT();
+-------------+
| ROW_COUNT() |
+-------------+
|      2      |
+-------------+

4. Replace

If there is no conflict, insert directly , In case of conflict, delete and then insert .

Two .Retrieve

select Column

 1. Full column query

However, it is generally not recommended to use * Make a full column query .

2. Specified column query

 

3. Query field expression  

3.1 Fields not included in the table

 3.2 The expression contains a field

3.3 The expression contains multiple fields

 

 4. Specify an alias for the query results

5. The results were de duplicated

 select distinct table_ele_name from table_name 

WHERE Conditions

Comparison operator ：

Operator                                                                     explain
>, >=, <, <=                                   Greater than , Greater than or equal to , Less than , Less than or equal to
=                                              be equal to ,NULL unsafe , for example NULL = NULL The result is NULL
<=>                                      be equal to ,NULL Security , for example NULL <=> NULL The result is TRUE(1)
!=, <>                                                                It's not equal to
BETWEEN a0 AND a1             Range match ,[a0, a1], If a0 <= value <= a1, return TRUE(1)
IN (option, ...)                              If it is option Any one of , return TRUE(1)
IS NULL                                                            yes NULL
IS NOT NULL                                                  No NULL
LIKE                      Fuzzy matching .% Denotes any number of （ Include 0 individual ） Any character ;_ Represents any character

Logical operators ：

Operator                                                                     explain
AND                                More than one condition must be TRUE(1), The result is TRUE(1)
OR                                  Any one of the conditions is TRUE(1), The result is TRUE(1)
NOT                                Condition is TRUE(1), The result is FALSE(0)

This is the full meter

1. We find that our math score is less than 80 Classmate

2. Next, let's look for Chinese scores in 【75,85】 Classmate

  The above uses and Perform conditional splicing , Next use between...and...

 3. The grade of Chinese is 80.81 or 82 Classmate

 

  What we use above is or, Let's use in Conditions

 4. A classmate surnamed Zhang

  above % Match any number of arbitrary characters , If you use _, Match an arbitrary character . Let's insert data to verify .

 5. Students whose Chinese scores are better than those in mathematics

 6. The total score is lower than 150 Classmate

7. The total score is greater than 150 A classmate whose surname is not Zhang （ Li Si ） , So let's verify that

Sorting results

-- ASC In ascending order （ From small to large ）
-- DESC For the descending order （ From big to small ）
-- The default is ASC

1. We will display the data in the table in ascending order of math scores

 2. We will display the data in the table in descending order of math scores

 3. In ascending order of language , Display in descending mathematical order

When collation conflicts , First follow the sorting rules written first .

4. Query students and total scores , Display from high to low （ Descending ）

 

5. Check the results of students surnamed Zhang , The results are displayed from high to low according to math scores （ Descending ） 

Filter result page

-- from s Start , Screening n Bar result , More definite than the second usage , It is recommended to use
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT n OFFSET s;

To better watch , Let's insert another data

3、 ... and .Update

1. Change Zhang San's score to 85

 

 2. Li Si's Chinese , The math scores were changed to 85

 3. Add to the last one 10 branch

We can see that the last one is 130 branch （ As a result of mistakes 130 Calculation ） 

 4. Double the Chinese scores of all the students

Four .delete

Delete data in table

1. Delete Zhang San's data

2. Next, we delete the entire table data （ Use with caution ）

  Truncation table

 

5、 ... and . Insert query results

Create a repeating table

  Create a non repeating table

6、 ... and . Aggregate functions

function                                                                         explain
COUNT([DISTINCT] expr)                        Returns the of the queried data Number
SUM([DISTINCT] expr)                            Returns the of the queried data The sum of the , It's not that numbers don't make sense
AVG([DISTINCT] expr)                            Returns the of the queried data Average , It's not that numbers don't make sense
MAX([DISTINCT] expr)                            Returns the of the queried data Maximum , It's not that numbers don't make sense
MIN([DISTINCT] expr)                              Returns the of the queried data minimum value , It's not that numbers don't make sense

1. Check how many students there are in the class

 2. View the... Collected in the table qq Number