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We can find all the triggers associated with a particular table with the help of the following query −mysql> Select * from INFORMATION_SCHEMA.TRIGGERS WHERE TRIGGER_SCHEMA = 'query'AND EVENT_OBJECT_TABLE = 'Student_info'\G *************************** 1. row *************************** TRIGGER_CATALOG: def TRIGGER_SCHEMA: query TRIGGER_NAME: studentinfo_after_delete EVENT_MANIPULATION: DELETE EVENT_OBJECT_CATALOG: def EVENT_OBJECT_SCHEMA: query EVENT_OBJECT_TABLE: student_info ACTION_ORDER: 1 ACTION_CONDITION: NULL ... Read More
The Main method is the entry point for all C# programs. It states what the class does when executed.The valid variant of Main() is −static void Main(string[] argsHere,static − the object is not needed to access static membersvoid − return type of the methodMain − entry point for any C# program. Program execution begins here.string[] args − for command line arguments in C#.ExampleHere is an example −using System; namespace Program { public class Demo { public static void Main(string[] args) { Console.WriteLine("Welcome!"); Console.ReadKey(); } } }
Main ThreadThe first thread to be executed in a process is called the main thread. When a C# program starts execution, the main thread is automatically created.Child ThreadThe threads created using the Thread class are called the child threads of the main thread.Here is an example showing how to create a main and child thread −Example Live Demousing System; using System.Threading; namespace Demo { class Program { static void Main(string[] args) { Thread th = Thread.CurrentThread; th.Name = "MainThread"; Console.WriteLine("This is {0}", th.Name); Console.ReadKey(); } } }OutputThis is MainThread
To initialize a dictionary to an empty dictionary, use the Clear() method. It clears the dictionary and forms it as empty.dict.Clear();After that, use the Dictionary count property to check whether the list is empty or not −if (dict.Count == 0) { Console.WriteLine("Dictionary is empty!"); }Let us see the complete code −Example Live Demousing System; using System.Collections.Generic; using System.Linq; namespace Demo { public class Program { public static void Main(string[] args) { var dict = new Dictionary < string, string > (); dict.Clear(); if (dict.Count == 0) { Console.WriteLine("Dictionary is empty!"); } } } }OutputDictionary is empty!
Actually ‘FOR EACH ROW’ means for each of the matched rows that get either updated or deleted. In other words, we can say that trigger is not applied to each row, it just says to execute the trigger body for each affected table row. We can illustrate this by the following example −ExampleIn this example, we are creating two tables, Sample and Sample_rowaffected, as follows −mysql> Create table Sample(id int, value varchar(20)); Query OK, 0 rows affected (0.47 sec) mysql> Insert into Sample(id, value) values(100, 'same'), (101, 'Different'), (500, 'excellent'), (501, 'temporary'); Query OK, 4 rows affected (0.04 sec) ... Read More
If there is ‘empty set’ in the result set of MySQL query then it means that MySQL is returning no rows and no error also in the query. It can be understood with the help of the following example −mysql> Select * from Student_info WHERE Name = 'ABCD'; Empty set (0.00 sec)We can see the empty set and execution time as output. It means that the query is correct but the MySQL table is not having the name ‘ABCD’.
With the help of the SHOW TRIGGERS statement, we can list all the triggers in a particular database. It can be illustrated with the help of the following example −Examplemysql> Show Triggers\G *************************** 1. row *************************** Trigger: trigger_before_delete_sample Event: DELETE Table: sample Statement: BEGIN SET @count = if (@count IS NULL, 1, (@count+1)); INSERT INTO sample_rowaffected values (@count); END Timing: BEFORE Created: 2017-11-21 12:31:58.70 sql_mode: ONLY_FULL_GROUP_BY, STRICT_TRANS_TABLES, NO_ZERO_IN_DATE, NO_ZERO_DATE, ERR OR_FOR_DIVISION_BY_ZERO, NO_AUTO_CREATE_USER, NO_ENGINE_SUBSTITUTION Definer: root@localhost character_set_client: cp850 collation_connection: cp850_general_ci Database Collation: latin1_swedish_ci *************************** 2. ... Read More
With the help of \c command, we can discard a MySQL statement in the middle of its processing. Consider the following example in which in the middle of the statement we want to discard it then we use \c option for it −mysql> Select * -> from\c mysql>The above query shows that after using \c option MySQL discards the statement and returns to the prompt.
We must have to understand the advantages, disadvantages, and restrictions of using MySQL triggers so that we can use it effectively.AdvantagesFollowings are the advantages of using MySQL triggers −Integrity of data − With the help of MySQL trigger we can check the integrity of data in the table. In other words, MySQL triggers are the alternative way to check the integrity of data.Useful for catching errors − MySQL triggers can catch errors in business logic in the database layer.Alternative way to run scheduled tasks − Actually by using MySQL triggers we do not have to wait to run the scheduled tasks because ... Read More
With the help of following MySQL statement, we can use SELECT to calculate the expression −SELECT expression;The above statement is having no reference to any table. Followings are some examples −mysql> Select 10 DIV 5; +----------+ | 10 DIV 5 | +----------+ | 2 | +----------+ 1 row in set (0.06 sec) mysql> Select 10*5; +------+ | 10*5 | +------+ | 50 | +------+ 1 row in set (0.00 sec) mysql> Set @var1 = 100, @var2 = 200; Query OK, 0 rows affected (0.00 sec) mysql> Select @Var1+@Var2; +-------------+ | @Var1+@Var2 | +-------------+ | 300 | +-------------+ 1 row in set (0.00 sec)