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MySQL Articles
Page 68 of 355
How can MySQL handle the errors during trigger execution?
Suppose if an error occurs during trigger execution then MySQL can handle it as follows −If a BEFORE trigger fails, the operation on the corresponding row is not performed.A BEFORE trigger is activated by the attempt to insert or modify the row, regardless of whether the attempt subsequently succeeds.An AFTER trigger is executed only if any BEFORE triggers and the row operation execute successfully.An error during either a BEFORE or AFTER trigger results in failure of the entire statement that caused trigger invocation.For transactional tables, failure of a statement should cause a rollback of all changes performed by the statement. ...
Read MoreHow BEFORE UPDATE triggers can be used to emulate CHECK CONSTRAINTnfor updating values in the table?
As we know that MySQL supports foreign key for referential integrity but it does not support CHECK constraint. But we can emulate them by using triggers. It can be illustrated with the help of an example given below −ExampleSuppose we have a table named ‘car’ which can have the fix syntax registration number like two letters, a dash, three digits, a dash, two letters as follows −mysql> Create table car (number char(9)); Query OK, 0 rows affected (0.32 sec)Creating BEFORE UPDATE trigger to emulate CHECK CONSTRAINT for updating the values −Now, suppose if we will try to update the table ...
Read MoreWhat are the benefits of using MySQL views as compared to selecting data directly from MySQL base tables?
As we know that views are definitions built on the top of other tables or views and stored in the database. Followings are benefits of using MySQL views as compared to selecting data directly from MySQL base tablesSimplify data accessThe use of views simplifies the data access because of the following reasons −A view can be used to perform a calculation and display its result. For example, a view definition that invokes aggregate functions can be used to display a summary.With the help of views, we can select a restricted set of rows by means of an appropriate WHERE clause ...
Read MoreWhat are the prerequisites before starting writing and using MySQL views?
MySQL VersionAs we know that MySQL 5 introduced views, hence, first of all, we need to check for the version of MySQL before starting writing and using stored procedures. It can be done with the following query −mysql> Select VERSION(); +-----------+ | VERSION() | +-----------+ | 5.7.20 | +-----------+ 1 row in set (0.10 sec)Privileges for current userActually CREATE VIEW statement requires the CREATE VIEW privilege. Privileges for the current user can be checked with the following query −mysql> SHOW PRIVILEGESSelecting a databaseBefore creating a view we must have to select a database from the available databases. ...
Read MoreWhat are the limitations of using MySQL views?
In spite of various benefits of using views there are following limitations on using MySQL views − Can’t create an index of views − In MySQL, we cannot create an index on views. It is because indexes are not utilized when we query data against the views. MySQL invalidates the view − Suppose, if we drop or rename tables to which a view references, rather than issuing an error MySQL invalidate the view. We can use the CHECK TABLE statement to check whether the view is valid or not. MySQL views cannot be updateable in some situations − Actually, the ...
Read MoreHow can I move an existing MySQL event to another database?
It can be done with the help of ALTER EVENT statement too. We need to use the combination of database name and event name along with the RENAME keyword. To illustrate it we are having the following example in which we are moving the event named ‘hello_renamed’ from ‘query’ database to ‘tutorial’ database −Examplemysql> ALTER EVENT query.hello_renamed RENAME to tutorials.hello_renamed; Query OK, 0 rows affected (0.00 sec)To confirm that event has been moved to database ‘tutorials’ we can try to delete the event with an old name, MySQL will throw an error as follows −mysql> DROP event hello_renamed; ERROR 1539 ...
Read MoreHow can we simulate the MySQL INTERSECT query?
Since we cannot use INTERSECT query in MySQL, we will use IN operator to simulate the INTERSECT query. It can be understood with the help of the following example −ExampleIn this example, we are two tables namely Student_detail and Student_info having the following data −mysql> Select * from Student_detail; +-----------+---------+------------+------------+ | studentid | Name | Address | Subject | +-----------+---------+------------+------------+ | 101 | YashPal | Amritsar | History | | 105 | Gaurav | Chandigarh | Literature | | 130 | Ram | Jhansi ...
Read MoreWhat are the different status variables in MySQL which provide us the countsnof event-related operations?
Followings are the status variables in MYSQL which provide us the counts of event-related operations −Com_create_event It provides us the number of CREATE EVENT statements executed since the last server restart.Com_alter_event − It provides us the number of ALTER EVENT statements executed since the last server restart.Com_drop_event − It provides us the number of DROP EVENT statements executed since the last server restart.Com_show_create_event − It provides us the number of SHOW CREATE EVENT statements executed since the last server restart.Com_show_events − It provides us the number of SHOW EVENTS statements executed since the last server restart.
Read MoreHow can we simulate the MySQL MINUS query?
Since we cannot use the MINUS query in MySQL, we will use JOIN to simulate the MINUS query. It can be understood with the help of the following example −ExampleIn this example, we are two tables namely Student_detail and Student_info having the following data −mysql> Select * from Student_detail; +-----------+---------+------------+------------+ | studentid | Name | Address | Subject | +-----------+---------+------------+------------+ | 101 | YashPal | Amritsar | History | | 105 | Gaurav | Chandigarh | Literature | | 130 | Ram | Jhansi ...
Read MoreHow can we get the count of all MySQL event-related operations collectively?
With the help of SHOW STATUS statement, we can get the count of MySQL event-related operations. It can be used as follows −mysql> SHOW STATUS LIKE '%event%'; +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | Com_alter_event | 16 | | Com_create_event | 6 | | Com_drop_event | 4 | | Com_show_binlog_events | 0 | | Com_show_create_event | 0 | | Com_show_events | 4 | | Com_show_relaylog_events | 0 | +--------------------------+-------+ 7 rows in set (0.17 sec)
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