sql

Author: Tayeb-Ahmed-TAS

SELECT CLAUSE 2/EXISTS Operator.sql

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+-- The SQL EXISTS operator tests the existence of any value in a subquery
+-- i.e. it executes the outer SQL query only if the subquery is not NULL (empty result-set).
+
+-- Syntax:
+/*
+SELECT column1, column2, ...
+FROM table_name
+WHERE EXISTS (subquery);
+*/
+
+-- Example: select all customers from the customers table who have placed an order
+
+SELECT id, name
+FROM customers
+WHERE EXISTS(SELECT order_id
+             FROM order_details
+             WHERE customers.id = order_details.customer_id);
+
+-- Example: -- select customer id , name AND country of customers whose order amount is less than 12000
+
+SELECT id, name, country
+FROM customers
+WHERE EXISTS(SELECT order_id
+             FROM order_details
+             WHERE customers.id = order_details.customer_id
+               AND price < 12000);

SELECT CLAUSE 2/NOT EXISTS.sql

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+-- We can also use the NOT operator to inverse the working of the EXISTS clause.
+-- The SQL command executes if the subquery returns an empty result-set.
+
+-- -- Example: select all customers from the customers table who have not placed an order
+
+SELECT id, name
+FROM customers
+WHERE NOT EXISTS(SELECT order_id
+                 FROM order_details
+                 WHERE customers.id = order_details.customer_id);

SQL JOINS/SQL JOINS.sql

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-select *,
-       case
-           when age > any (select age from teachers) then 'Declined'
-           else 'Approved'
-           end as Status
-from students;