diff --git a/sql-queries-6/add-foreign-key-existing-table/add-foreign-key-mssql.sql b/sql-queries-6/add-foreign-key-existing-table/add-foreign-key-mssql.sql
new file mode 100644
index 00000000..89fb908c
--- /dev/null
+++ b/sql-queries-6/add-foreign-key-existing-table/add-foreign-key-mssql.sql
@@ -0,0 +1,17 @@
+--adding a new column for foreign key
+ALTER TABLE Student
+ADD department_id INT;
+
+--adding a foreign key to the student table
+ALTER TABLE Student
+ADD CONSTRAINT fk_student_department
+FOREIGN KEY (department_id) REFERENCES Department(id);
+
+--adding unique constraint in the department table
+ALTER TABLE Department
+ADD CONSTRAINT uq_department UNIQUE (id, name);
+
+--adding a foreign key to multiple columns
+ALTER TABLE Course
+ADD CONSTRAINT fk_course_department FOREIGN KEY (department_id, name)
+REFERENCES Department (id, name);
diff --git a/sql-queries-6/add-foreign-key-existing-table/add-foreign-key-mysql.sql b/sql-queries-6/add-foreign-key-existing-table/add-foreign-key-mysql.sql
new file mode 100644
index 00000000..33a09c53
--- /dev/null
+++ b/sql-queries-6/add-foreign-key-existing-table/add-foreign-key-mysql.sql
@@ -0,0 +1,17 @@
+--adding a new column for foreign key
+ALTER TABLE Student
+ADD COLUMN department_id INT;
+
+-- adding a foreign key to an existing table
+ALTER TABLE Student
+ADD CONSTRAINT fk_student_department
+FOREIGN KEY (department_id) REFERENCES Department(id);
+
+--adding unique constraints in the department table
+ALTER TABLE Department
+ADD CONSTRAINT uq_department UNIQUE (id, name);
+
+--adding a foreign key to multiple columns
+ALTER TABLE Course
+ADD CONSTRAINT fk_course_department FOREIGN KEY (department_id, name)
+REFERENCES Department (id, name);
diff --git a/sql-queries-6/add-foreign-key-existing-table/add-foreign-key-postgresql.sql b/sql-queries-6/add-foreign-key-existing-table/add-foreign-key-postgresql.sql
new file mode 100644
index 00000000..9b103e4a
--- /dev/null
+++ b/sql-queries-6/add-foreign-key-existing-table/add-foreign-key-postgresql.sql
@@ -0,0 +1,13 @@
+--adding a new column
+ALTER TABLE Student
+ADD COLUMN department_id INT;
+
+--adding a foreign key constraint to an existing table
+ALTER TABLE Student
+ADD CONSTRAINT fk_student_department
+FOREIGN KEY (department_id) REFERENCES Department(id);
+
+--adding a foreign key constraint on multiple columns
+ALTER TABLE Course 
+ADD CONSTRAINT fk_course_department 
+FOREIGN KEY (department_id, name) REFERENCES Department (id, name);