[SQL-519] What is a trigger in SQL

data-manipulation/triggers/triggers-mysql.sql

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+CREATE TABLE IF NOT EXISTS Student_audit_log (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  student_id INT,
+  action VARCHAR(255) NOT NULL,
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+DROP TRIGGER IF EXISTS after_student_insert;
+DROP TRIGGER IF EXISTS after_student_update;
+DROP TRIGGER IF EXISTS after_student_delete;
+
+DELIMITER //
+CREATE TRIGGER after_student_insert
+    AFTER INSERT ON Student
+    FOR EACH ROW
+BEGIN
+    INSERT INTO student_audit_log(student_id, action)
+    VALUES (NEW.id, 'INSERT');
+END//
+
+CREATE TRIGGER after_student_update
+    AFTER UPDATE ON Student
+    FOR EACH ROW
+BEGIN
+    INSERT INTO student_audit_log(student_id, action)
+    VALUES (NEW.id, 'UPDATE');
+END//
+
+CREATE TRIGGER after_student_delete
+    AFTER DELETE ON Student
+    FOR EACH ROW
+BEGIN
+    INSERT INTO student_audit_log(student_id, action)
+    VALUES (OLD.id, 'DELETE');
+END//
+DELIMITER ;

data-manipulation/triggers/triggers-pg.sql

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+CREATE TABLE IF NOT EXISTS Student_audit_log (
+  id SERIAL PRIMARY KEY,
+  student_id INT,
+  action VARCHAR(255) NOT NULL,
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE OR REPLACE FUNCTION log_student_changes()
+RETURNS TRIGGER AS $$
+BEGIN
+    IF TG_OP = 'INSERT' THEN
+        INSERT INTO Student_audit_log(student_id, action)
+        VALUES (NEW.id, 'INSERT');
+        RETURN NEW;
+    ELSIF TG_OP = 'UPDATE' THEN
+        INSERT INTO Student_audit_log(student_id, action)
+        VALUES (NEW.id, 'UPDATE');
+        RETURN NEW;
+    ELSIF TG_OP = 'DELETE' THEN
+        INSERT INTO Student_audit_log(student_id, action)
+        VALUES (OLD.id, 'DELETE');
+    RETURN OLD;
+END IF;
+RETURN NULL;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER student_audit_trigger
+    AFTER INSERT OR UPDATE OR DELETE ON Student
+    FOR EACH ROW EXECUTE FUNCTION log_student_changes();
+
+CREATE OR REPLACE FUNCTION log_bulk_deletion_student()
+RETURNS TRIGGER AS $$
+BEGIN
+INSERT INTO Student_audit_log (action)
+VALUES ('BULK_DELETION');
+RETURN NULL;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER log_bulk_deletion_student_trigger
+    AFTER DELETE ON Student
+    FOR EACH STATEMENT EXECUTE FUNCTION log_bulk_deletion_student();
+
+CREATE OR REPLACE VIEW Department_view AS
+SELECT * FROM Department;
+
+CREATE OR REPLACE FUNCTION prevent_department_delete()
+RETURNS TRIGGER AS $$
+BEGIN
+RETURN NULL;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER prevent_department_deletion_trigger
+    INSTEAD OF DELETE ON Department_view
+    FOR EACH ROW
+    EXECUTE FUNCTION prevent_department_delete();

data-manipulation/triggers/triggers-sqlserver.sql

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+IF NOT EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[Student_audit_log]') AND type in (N'U'))
+BEGIN
+  CREATE TABLE Student_audit_log (
+    id INT IDENTITY(1,1) PRIMARY KEY,
+    student_id INT,
+    action VARCHAR(255) NOT NULL,
+    created_at DATETIME DEFAULT GETDATE()
+);
+END
+
+-- Drop existing triggers if they exist
+IF EXISTS (SELECT * FROM sys.triggers WHERE name = 'trg_student_audit')
+DROP TRIGGER trg_student_audit;
+GO
+
+CREATE TRIGGER trg_student_audit
+    ON Student
+    AFTER INSERT, UPDATE, DELETE
+    AS
+BEGIN
+    SET NOCOUNT ON;
+
+
+    IF EXISTS (SELECT * FROM inserted) AND NOT EXISTS (SELECT * FROM deleted)
+    BEGIN
+        INSERT INTO student_audit_log (student_id, action)
+        SELECT id, 'INSERT'
+        FROM inserted;
+    END
+
+    IF EXISTS (SELECT * FROM inserted) AND EXISTS (SELECT * FROM deleted)
+    BEGIN
+        INSERT INTO student_audit_log (student_id, action)
+        SELECT id, 'UPDATE'
+        FROM inserted;
+    END
+
+    IF EXISTS (SELECT * FROM deleted) AND NOT EXISTS (SELECT * FROM inserted)
+    BEGIN
+        INSERT INTO student_audit_log (student_id, action)
+        SELECT id, 'DELETE'
+        FROM deleted;
+    END
+END
+GO;
+
+CREATE TRIGGER prevent_department_deletion_trigger
+    ON Department
+    INSTEAD OF DELETE
+AS
+BEGIN
+  RETURN;
+END;

mysql-queries/README.md

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+### Relevant Articles:
+-  [How to Find the Engine for a Specific Table in MySQL?](https://www.baeldung.com/sql/mysql-find-table-storage-engine)
+-  [Changing a Table’s Storage Engine in MySQL](https://www.baeldung.com/sql/mysql-change-table-storage-engine)
+-  [Pattern Matching in MySQL](https://www.baeldung.com/sql/mysql-pattern-matching)
+-  [Comments in MySQL](https://www.baeldung.com/sql/mysql-comments-single-line-multiline-hint)

mysql-queries/change-table-engine-in-mysql/changing-storage-engine.sql

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+ALTER TABLE Student ENGINE=MyISAM;
+
+SELECT 
+    engine 
+FROM 
+    information_schema.tables
+WHERE 
+    table_schema = 'University' 
+    AND table_name = 'Student';
+
+ALTER TABLE Student ENGINE=InnoDB;

mysql-queries/change-table-engine-in-mysql/checking-storage-engine.sql

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+SELECT 
+    engine
+FROM
+    information_schema.tables
+WHERE
+    table_schema = 'University'
+    AND table_name = 'Student';
+
+SHOW TABLE STATUS LIKE 'Student';

mysql-queries/change-table-engine-in-mysql/show-storage-engine.sql

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+SHOW ENGINES;

mysql-queries/comments-mysql/comments.mysql

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+-- This query returns all Students in the University
+SELECT * FROM Student;
+SELECT * FROM Student -- WHERE NAME = 'John';
+
+SELECT * FROM Student WHERE NAME /* wild card matching for name */ LIKE '%John%';
+
+SELECT * FROM Student WHERE NAME 
+/* wild card matching for name. 
+Change it based on the requirement 
+*/ 
+LIKE '%John%';
+
+SELECT /*! STRAIGHT_JOIN */ * FROM Student /*+ USE INDEX (PRIMARY) */ WHERE id = 1001;

mysql-queries/detect-locked-table-mysql/checking-lock-in-performance_schema.metadata_locks.sql

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+SELECT * FROM performance_schema.metadata_locks 
+WHERE OBJECT_TYPE = 'USER LEVEL LOCK';

mysql-queries/detect-locked-table-mysql/locking-table.sql

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+LOCK TABLES Student WRITE;
+

mysql-queries/detect-locked-table-mysql/unlock-table.sql

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+UNLOCK TABLES;
+
+SELECT RELEASE_LOCK('Student');

mysql-queries/detect-locked-table-mysql/using-show-open-table.sql

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+SHOW OPEN TABLES WHERE `Table` LIKE 'Student' AND `Database` LIKE 'University' AND In_use > 0;
+
+SHOW OPEN TABLES WHERE In_use > 0;

mysql-queries/find-storage-engine/script.sql

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+-- specify the database
+USE University;
+
+-- find the engine of University.Student
+-- using the information_schema.tables
+SELECT engine
+FROM information_schema.tables
+WHERE table_schema = 'University'
+  AND table_name = 'Student';
+
+-- find the engine of all the tables in University
+-- using the information_schema.tables
+SELECT table_name, engine
+FROM information_schema.tables
+WHERE table_schema = 'University'
+  AND table_type = 'BASE TABLE'
+  AND table_name = 'Student';
+
+-- find the engine of University.Student
+-- using the information_schema.tables
+SHOW TABLE STATUS 
+FROM University
+WHERE name = 'Student';

mysql-queries/mysql-images/images.sql

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+-- SQL Script for the article Images in MySQL
+
+-- Change schema to University
+USE University;
+
+-- Check max image size you can upload
+SHOW VARIABLES LIKE 'max_allowed_packet';
+SHOW VARIABLES LIKE 'secure_file_priv';
+
+-- Grant access
+GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION;
+FLUSH PRIVILEGES;
+
+-- DIRECT STORAGE
+
+-- Create Tables
+CREATE TABLE DepartmentComplete
+(
+    id INT PRIMARY KEY NOT Null,
+    name VARCHAR (50),
+    code VARCHAR (4),
+    logo BLOB,
+    UNIQUE (id)
+);
+
+CREATE TABLE StudentComplete
+(
+    id INT PRIMARY KEY NOT null,
+    name VARCHAR (60),
+    national_id BIGINT NOT Null, 
+    birth_date DATE,
+    enrollment_date DATE,
+    graduation_date DATE,
+    gpa FLOAT,
+    profile_pic BLOB,
+    UNIQUE (id)
+);
+
+-- Adding data for DepartmentComplete 
+INSERT INTO DepartmentComplete (id, name, code, logo) VALUES
+  (1, 'Computer Science', 'CS', LOAD_FILE('/usr/local/mysql/data/cs_logo.png')),
+  (2, 'Electronics and Communications', 'EC', LOAD_FILE('/usr/local/mysql/data/ece_logo.png')),
+  (3, 'Social Sciences', 'SS', LOAD_FILE('/usr/local/mysql/data/ss_logo.png')),
+  (4, 'Computational Biology', 'CB', LOAD_FILE('/usr/local/mysql/data/cb_logo.png')),
+  (5, 'Mathematics', 'MA', LOAD_FILE('/usr/local/mysql/data/ma_logo.png'));
+
+-- Adding data for StudentComplete
+INSERT INTO StudentComplete (id, name, national_id, birth_date, enrollment_date, graduation_date, gpa, profile_pic) VALUES
+  (1001, 'John Liu', 123345566, '2001-04-05', '2020-01-15', '2024-06-15', 4, LOAD_FILE('/usr/local/mysql/data/john_pp.jpeg')),
+  (1003, 'Rita Ora', 132345166, '2001-01-14', '2020-01-15', '2024-06-15', 4.2, LOAD_FILE('/usr/local/mysql/data/rita_pp.jpeg')),
+  (1007, 'Philip Lose', 321345566, '2001-06-15', '2020-01-15', '2024-06-15', 3.8, LOAD_FILE('/usr/local/mysql/data/philip_pp.jpeg')),
+  (1010, 'Samantha Prabhu', 3217165566, '2001-03-21', '2020-01-15', '2024-06-15', 4.9, LOAD_FILE('/usr/local/mysql/data/cs_logo.png')),
+  (1011, 'Vikas Jain', 321345662, '2001-7-18', '2020-01-15', NULL, 3.3, LOAD_FILE('/usr/local/mysql/data/vikas_pp.jpeg'));
+
+SELECT * from DepartmentComplete;
+
+SELECT * from StudentComplete;
+
+-- INDIRECT STORAGE
+-- Alter Table DepartmentComplete to drop logo
+ALTER TABLE DepartmentComplete
+DROP COLUMN logo;
+
+-- Alter Table DepartmentComplete to add logo_path
+ALTER TABLE DepartmentComplete 
+ADD logo_path VARCHAR (256);
+
+-- Alter Table StudentComplete to drop profile_pic
+ALTER TABLE StudentComplete
+DROP COLUMN profile_pic;
+
+-- Alter Table StudentComplete to add profile_pic_path
+ALTER TABLE StudentComplete 
+ADD profile_pic_path VARCHAR (256);
+
+
+-- Update data 
+UPDATE DepartmentComplete SET logo_path='/usr/local/mysql/data/cs_logo.png' WHERE code ='CS'; 
+UPDATE DepartmentComplete SET logo_path='/usr/local/mysql/data/ece_logo.png' WHERE code ='EC'; 
+UPDATE DepartmentComplete SET logo_path='/usr/local/mysql/data/ss_logo.png' WHERE code ='SS'; 
+UPDATE DepartmentComplete SET logo_path='/usr/local/mysql/data/cb_logo.png' WHERE code ='CB'; 
+UPDATE DepartmentComplete SET logo_path='/usr/local/mysql/data/ma_logo.png' WHERE code ='MA'; 
+
+UPDATE StudentComplete SET profile_pic_path='/usr/local/mysql/data/john_pp.jpeg' WHERE name ='John Liu'; 
+UPDATE StudentComplete SET profile_pic_path='/usr/local/mysql/data/rita_pp.jpeg' WHERE name ='Rita Ora'; 
+UPDATE StudentComplete SET profile_pic_path='/usr/local/mysql/data/philip_pp.jpeg' WHERE name ='Philip Lose'; 
+UPDATE StudentComplete SET profile_pic_path='/usr/local/mysql/data/samantha_pp.jpeg' WHERE name ='Samantha Prabhu'; 
+UPDATE StudentComplete SET profile_pic_path='/usr/local/mysql/data/vikas_pp.jpeg' WHERE name ='Vikas Jain'; 
+
+-- END

mysql-queries/mysql-pattern-matching/pattern-matching.sql

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+-- Change schema to University
+USE University;
+
+-- LIKE
+SELECT * FROM Student 
+WHERE name LIKE LOWER ('R%');
+
+SELECT * FROM Student 
+WHERE name LIKE BINARY 'r%';
+
+SELECT * FROM Student 
+WHERE name LIKE '_________';
+
+-- REGEXP_LIKE
+SELECT * FROM Student 
+WHERE REGEXP_LIKE(name, '^R');
+
+SELECT * FROM Student 
+WHERE REGEXP_LIKE(name, '^r', 'c');
+
+SELECT Department.name, Course.name 
+FROM Department LEFT JOIN Course ON Department.id = Course.department_id 
+WHERE REGEXP_LIKE(Course.name, 'to') 
+AND Department.code="CS";
+
+-- LOCATE
+SELECT * FROM Student 
+WHERE LOCATE('R', name, 1)=1;
+
+SELECT * FROM Department 
+WHERE LOCATE('a', name) < 0 
+AND LOCATE('a', name) < 0 
+AND LOCATE('e', name) < 0 
+AND LOCATE('i', name) < 0 
+AND LOCATE('o', name) < 0 
+AND LOCATE('u', name) < 0;