Recommended video tutorial resources for you to play with MySQL in six days

MySQL is the most popular open source relational database management system (RDBMS). "Video Tutorial on MySQL in Six Days" will help you quickly master the basic knowledge of MySQL and easily use MySQL database to store and manage large amounts of data. Today, the combination of mysql and php is absolutely perfect. Many large websites also use the mysql database. The development prospects of mysql are very bright!

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##The teacher’s teaching style:

Teachers' lectures are simple and in-depth, with clear organization, layer-by-layer analysis, interlocking links, rigorous argumentation, and rigorous structure. They use the logical power of thinking to attract students' attention and use reason to control the classroom teaching process. By listening to the teacher's lectures, students not only learn knowledge, but also receive thinking training, and are also influenced and infected by the teacher's rigorous academic attitude.

The more difficult point in this video is the trigger:

MySQL includes support for triggers. A trigger is a database object related to table operations. When a specified event occurs on the table where the trigger is located, the object will be called, that is, the operation event of the table triggers the execution of the trigger on the table.

Create trigger

In MySQL, the syntax for creating a trigger is as follows:

The code is as follows:

CREATE TRIGGER trigger_name
trigger_time
trigger_event ON tbl_name
FOR EACH ROW
trigger_stmt

Among them:

trigger_name: identifies the trigger The trigger name is specified by the user;

trigger_time: identifies the triggering time, the value is BEFORE or AFTER;
trigger_event: identifies the trigger event, the value is INSERT, UPDATE or DELETE;
tbl_name: identifies the trigger creation The table name, that is, on which table the trigger is created;
trigger_stmt: Trigger program body, which can be a SQL statement, or multiple statements contained in BEGIN and END.

It can be seen that 6 types of triggers can be created, namely: BEFORE INSERT, BEFORE UPDATE, BEFORE DELETE, AFTER INSERT, AFTER UPDATE, and AFTER DELETE.

Another limitation is that you cannot create two triggers of the same type on a table at the same time, so a maximum of 6 triggers can be created on a table.

trigger_event Detailed explanation

In addition to defining the basic operations of INSERT, UPDATE, and DELETE, MySQL also defines the LOAD DATA and REPLACE statements. These two statements can also cause the triggering of the above 6 types of triggers. .

The LOAD DATA statement is used to load a file into a data table, which is equivalent to a series of INSERT operations.

The REPLACE statement is generally very similar to the INSERT statement, except that when there is a primary key or unique index in the table, if the inserted data is consistent with the original primary key or unique index, the original data will be deleted first. Then add a new piece of data, that is, a REPLACE statement is sometimes equivalent to one.

The INSERT statement is sometimes equivalent to a DELETE statement plus an INSERT statement.

INSERT type trigger: The trigger is activated when a certain row is inserted, which may be triggered by INSERT, LOAD DATA, and REPLACE statements;

UPDATE type trigger: The trigger is activated when a certain row is changed, which may be triggered by the UPDATE statement. Trigger;
DELETE trigger: The trigger is activated when a certain row is deleted, and may be triggered by DELETE and REPLACE statements.

BEGIN … END Detailed explanation

In MySQL, the syntax of the BEGIN … END statement is:

BEGIN
[statement_list]
END

Among them, statement_list represents a list of one or more statements, and each statement in the list All must end with a semicolon (;).

In MySQL, the semicolon is the identifier of the end of the statement. When encountering a semicolon, it means that the statement has ended and MySQL can start execution. Therefore, the interpreter starts execution after encountering a semicolon in statement_list, and then reports an error because no END matching BEGIN is found.

The DELIMITER command will be used at this time (DELIMITER is the delimiter, the meaning of the separator). It is a command and does not require an end-of-statement identifier. The syntax is:

DELIMITER new_delemiter
new_delemiter can be set to one or more length symbols, the default is semicolon (;), we can modify it to other symbols, such as $:
DELIMITER $
The statement after this, If it ends with a semicolon, the interpreter will not react. Only when $ is encountered, the statement will be considered to have ended. Note that after using it, we should remember to modify it back.

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