MySQL advanced SQL statement of database (2)

One 、MySQL Union set 、 Intersection value 、 No intersection value 、case

1、 combine

• Put two SQL The results of the statement are combined , Two SQL The fields generated by the statement need to be of the same data type

（1）UNION

（2）UNION ALL

2、 Intersection value

• Take two. SQL The intersection of statement results
  
• There are no duplicate rows in the two tables , And when there are intersections, use
  

• Take two. SQL The intersection of statement results , And there's no repetition

3、 No intersection value

• Show the first SQL Result of statement , And with the second SQL Statement has no result of intersection , And there's no repetition
  

4、CASE

Two 、 Null value (NULL) and No value (’’) The difference between

• The length of no value is 0, Not taking up space ; and NULL The length of the value is NULL, It takes up space .
• IS NULL perhaps IS NOT NULL, It is used to judge whether the field is NULL Or not NULL, Can't find out if it's worthless .
• The judgment without value uses =’‘ perhaps <>’' To deal with it .<> The representative is not equal to .
• Through count() When specifying the number of rows in the field statistics , If you encounter NULL The value is automatically ignored , If there is no value, it will be added to the record for calculation .

3、 ... and 、MySQL Regular expressions of

Four 、 stored procedure

1、 The concept of stored procedure

• A stored procedure is a set of functions to accomplish SQL Statement set .
• 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

4、 Calling stored procedure

5、 View stored procedures

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 ）

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 .

8、 Control statement of stored procedure