High level application of SQL statements in MySQL database (II)

1、CREATE VIEW View
View ： Can be treated as a virtual table or stored query .
The difference between a view and a table is , There are actually stored data in the table , And a view is a structure built on a table , It doesn't actually store data itself .

The temporary table disappears automatically after the user exits or disconnects from the database , The view does not .

Views don't contain data , Just store its definition , Its purpose is to simplify complex queries . For example, you need to connect and query several tables , But also to carry out statistical sorting and other operations , Write SQL Statements can be cumbersome , Connect several tables with a view , Then query the view , Just like querying a table , Very convenient .

grammar ：CREATE VIEW View table name AS SELECT sentence ;
example :

select A.region,SUM(B.money) from REGION AS A inner join FARE AS B on A.site = B.site GROUP BY region;
create view ST AS select A.region,SUM(B.money) from REGION AS A inner join FARE AS B on A.site = B.site GROUP BY region;

show tables;
select * from ST;

drop view ST;
# Delete view table

2.UNION combine
Put two SQL The results of the statement are combined , Two SQL The fields generated by the statement need to be the same data type
UNION： There will be no duplicate data values for the generated results , And sort according to the order of the fields

SELECT sentence 1 UNION SELECT sentence 2;
or
SELECT sentence 1 UNION ALL SELECT sentence 2;
#UNION ALL： List the data values of the generated results , With or without repetition

example ：
select region from REGION union select money from FARE;
select site from REGION union select site from FARE;
select site from REGION union all select site from FARE;

3. Intersection value
Take two. SQL The intersection of statement results
example ：

select * from FARE;
select * from REGION;

select site from (select A.site from FARE A inner join REGION B on A.site = B.site) C group by C.site;

select site from FARE where site in (select site from REGION);

4. No intersection value
Show the first SQL Result of statement , And with the second SQL Statement has no result of intersection , It can't be repeated
example ：

select distinct site from FARE where (site) not in (select site from REGION);

select A.,B. from FARE A left join REGION B using(site);
select distinct site from FARE A left join REGION B using(site) where B.site is null;

5.CASE
yes SQL Used as a IF-THEN-ELSE And so on
usage ：

SELECT CASE （ Field name ）
WHEN Conditions 1 THEN result 1
WHEN Conditions 2 THEN result 2
……
ELSE result N
END
FROM Table name

# The condition can be a numerical value or a formula , And ELSE Clause is not required

example ：
select * from FARE;

mysql> select case site
-> when ‘najing’ then money + 500
-> when ‘tianjin’ then money - 500
-> else money * 2
-> end
-> A,site
-> from FARE;

3、 ... and 、 Regular expressions
The specific application is shown in the following table ：

Matching mode describe example
^ Match the start character of the text ‘^bd’ Match with bd Starting string
$ Match the end character of the text ‘qn$’ Match with qn a null-terminated string
. Match any single character ‘s.t’ Match any s and t A string with one character between
***** Match zero or more characters before it ‘fo*t’ matching t There is any one in front o

• Match preceding characters 1 Times or times ‘hom+’ Match with ho start , At least one back m String
  character string Match contains the specified string ‘clo’ The match contains clo String
  […] Match any character in the character set ‘[abc]’ matching a perhaps b perhaps c
  [^…] Match any characters that are not in brackets ‘[^ab]’ Match does not contain a perhaps b String
  {n} Match the previous string n Time ‘g{2}’ The match contains 2 individual g String
  {n,m} Match the previous string at least n Time , at most m Time ‘f{1,3}’ matching f least 1 Time , most 3 Time
  p1 p2 matching p1 or p2
  usage ：

SELECT Field FROM Table name WHERE Field REGEXP Matching mode

example ：
select * from FARE where site regexp ‘ ^[1]’;
select * from FARE where site regexp ‘[n]’;
select * from FARE where site regexp ‘jin|jing’;

Four 、 stored procedure
1. summary
A stored procedure is a set of functions to accomplish SQL Statement set
And I learned before Shell The function is almost , In essence, it is the reuse of code
Further details ：
In the process of using stored procedures, common or complex work is used in advance SQL The statement is written and stored with a specified name
This process is compiled and optimized and stored in the database server
When you need to use this stored procedure , Just call it
Stored procedures perform better than traditional SQL Faster , More efficient execution

2. Advantages of stored procedures
After one execution , The generated binary code will reside in the buffer , Improve execution efficiency
SQL Statement plus a collection of control statements , High flexibility
Store on the server side , When called by the client , Reduce network load
Can be called repeatedly , Can be modified at any time , Does not affect client calls
Can complete all database operations , You can also control the information access rights of the database

3. Create stored procedure
DELIMITER $$ # Change the closing sign of the statement from a semicolon ; Temporary modification , In case something goes wrong , You can customize
CREATE PROCEDURE XXX() # Create stored procedure , Custom procedure name ,（） With parameters
BEGIN # The process body takes the keyword BEGIN Start
select * from xxx; # Process style sentences
END$$ # The process body takes the keyword END ending
DELIMITER ; # Returns the ending symbol of the statement to a semicolon

example ：
mysql> delimiter $$
mysql> create procedure XCF()
-> begin
-> select * from FARE;
-> end $$
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;

4. Calling stored procedure
CALL XCF;

5. View stored procedures
usage ：

show create procedure [ database .] Stored procedure name ; # View the specific information of a stored process
show create procedure XXX;
show procedure status [like ‘%XXX%’] \G

example ：
show create procedure train_ticket.XCF\G
show procedure status like ‘%XCF%’\G

6. Parameters of stored procedure
IN Input parameters ： Indicates that the caller passes a value... To the procedure （ The incoming value can be literal or variable ）
OUT Output parameters ： Indicates that the procedure passes out a value to the caller ( Multiple values can be returned )（ Outgoing values can only be variables ）
INOUT Input/output parameter ： It means that the caller passes in a value to the procedure , It also indicates that the procedure passes out a value to the caller （ Values can only be variables ）
example ：

mysql> delimiter $$
mysql> create procedure XCF0(IN place char(20))
-> begin
-> select * from FARE where site=place;
-> end $$
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter $$
mysql> call XCF0(‘nanjing’);
-> $$
±--------±------±-----------+
| site | money | date |
±--------±------±-----------+
| nanjing | 2000 | 2021-02-07 |
±--------±------±-----------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;

7. Delete stored procedure
The method to modify the contents of stored procedures is to delete the original stored procedures , Then create a new stored procedure with the same name
usage ：

DROP PROCEDURE IF EXISTS Proc;
# Delete... Only if it exists , If the specified procedure does not exist , Then there will be a mistake

example ：
drop procedure if exists XCF0;

8. Control statement of stored procedure
8.1 Conditional statements （if）
mysql> DELIMITER $$
mysql> CREATE PROCEDURE XCF1(IN num int(10))
-> BEGIN
-> declare var int;
-> set var=num*2;
-> if var>=10 then
-> update FARE set money=money+1;
-> else
-> update FARE set money=money-1;
-> end if;
-> END $$
Query OK, 0 rows affected (0.00 sec)

mysql> DELIMITER ;

CALL XCF1(5);
CALL XCF1(4);
# Call procedure body
select money from FARE;

8.2 Loop statement （while）
Copy
mysql> create table xcf(id int(5));
Query OK, 0 rows affected (0.01 sec)

mysql> DELIMITER $$
mysql> CREATE PROCEDURE XCF2()
-> BEGIN
-> declare var int;
-> set var=0;
-> while var<9 do
-> insert into xcf values(var);
-> set var=var+1;
-> end while;
-> END $$
Query OK, 0 rows affected (0.00 sec)

mysql> CALL XCF2;
-> $$
Query OK, 1 row affected (0.01 sec)

mysql> select * from xcf;
-> $$
±-----+
| id |
±-----+
| 0 |
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
| 7 |
| 8 |
±-----+
9 rows in set (0.00 sec)

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