sql why use multiple tables code example

Example: sql query multiple tables

-- With JOIN
-- No row if id does not exist in t2
SELECT t1.name, t2.salary FROM t1 JOIN t2 on t1.id = t2.id;
-- A row with a NULL salary is returned if id does not exist in t2
SELECT t1.name, t2.salary FROM t1 LEFT OUTER JOIN t2 on t1.id = t2.id;

-- With UNION: distinct values
SELECT emp_name AS name from employees
UNION
SELECT cust_name AS name from customers;

-- With UNION ALL: keeps duplicates (faster)
SELECT emp_name AS name from employees
UNION ALL
SELECT cust_name AS name from customers;