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Introduction Subquery : A SELECT statement located inside another SQL statement (can be inside SELECT, FROM, WHERE, HAVING ). It supplies data for the main query. Nested Query : A term used to describe the structure of the query. When a Subquery resides inside a parent statement, this action is called nesting. Thus, Subquery can be viewed as the component (the child), while Nested query is the structural relationship (the parent containing the child). A statement that contains a subquery has its entire structure referred to as a nested query . Classification Non-correlated Subquery : This type of subquery runs completely independently of the parent statement. Postgres executes this subquery exactly once, using its result to apply to the parent statement. Example: SELECT name, salary FROM employees WHERE salary > (SELECT AVG(salary) FROM employees) Here, SELECT AVG(salary) FROM employees is an independent Subquery that only needs to run once to provide the value for the outer ...

Introduction The EXISTS clause is used to solve a single problem in data logic which is checking the existence of data based on a condition, without worrying about how many times it exists or what the specific value is. How it works Just like when using IN with EXISTS , Postgres will also automatically use Semi-Join algorithms (or Anti-Semi-Join for NOT EXISTS ) Semi Join (EXISTS) : Returns rows from the main table (left table) if there is at least one matching row in the sub-table. The optimizer stops searching as soon as it finds the first matching row for each record of the main table. Anti Join (NOT EXISTS) : Contrary to Semi Join , it returns rows from the main table if no matching rows are found in the sub-table. It still uses popular algorithms such as Nested Loop Semi Join, Hash Semi Join, Merge Semi Join with a Short-circuit mechanism, meaning that for each row value, it only needs to iterate until the first value is found, rather than finding all matching values like wh...