how to create mysql trigger and use real life

CREATE TABLE demo(d1 INT);
CREATE TABLE demo2(d2 INT);
CREATE TABLE demo3(d3 INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
CREATE TABLE demo4(
  a4 INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  b4 INT DEFAULT 0
);

delimiter |
//event before and after implement the table
CREATE TRIGGER demotrigger BEFORE INSERT ON demo1
  FOR EACH ROW BEGIN
    INSERT INTO demo2 SET d2 = NEW.d1;
    DELETE FROM demo3 WHERE d3 = NEW.d1;
    UPDATE demo4 SET b4 = b4 + 1 WHERE a4 = NEW.d1;
  END;
|

delimiter ;

INSERT INTO demo3 (d3) VALUES
  (NULL), (NULL), (NULL), (NULL), (NULL),
  (NULL), (NULL), (NULL), (NULL), (NULL);

INSERT INTO demo4 (a4) VALUES
  (0), (0), (0), (0), (0), (0), (0), (0), (0), (0);

Suppose that you insert the following values into table test1 as shown here:

what is main difference between xml and json

Data types	Does not provide any notion of data types. One must rely on XML Schema for adding type information.	Provides scalar data types and the ability to express structured data through arrays and objects.
Support for arrays	Arrays have to be expressed by conventions, for example through the use of an outer placeholder element that models the arrays contents as inner elements. Typically, the outer element uses the plural form of the name used for inner elements.	Native array support.
Support for objects	Objects have to be expressed by conventions, often through a mixed use of attributes and elements.	Native object support.
Null support	Requires use of xsi:nil on elements in an XML instance document plus an import of the corresponding namespace.	Natively recognizes the null value.
Comments	Native support and usually available through APIs.	Not supported.
Namespaces	Supports namespaces, which eliminates the risk of name collisions when combining documents. Namespaces also allow existing XML-based standards to be safely extended.	No concept of namespaces. Naming collisions are usually avoided by nesting objects or using a prefix in an object member name (the former is preferred in practice).
Formatting decisions	Complex. Requires a greater effort to decide how to map application types to XML elements and attributes. Can create heated debates whether an element-centric or attribute-centric approach is better.	Simple. Provides a much more direct mapping for application data. The only exception may be the absence of date/time literal.
Size	Documents tend to be lengthy in size, especially when an element-centric approach to formatting is used.	Syntax is very terse and yields formatted text where most of the space is consumed (rightly so) by the represented data.
Parsing in JavaScript	Requires an XML DOM implementation and additional application code to map text back into JavaScript objects.	No additional application code required to parse text; can use JavaScript's eval function.
Learning curve	Generally tends to require use of several technologies in concert: XPath, XML Schema, XSLT,XML Namespaces, the DOM, and so on.	Very simple technology stack that is already familiar to developers with a background in JavaScript or other dynamic programming languages.