By using joins, you can retrieve data from two or more tables based on logical relationships between the tables.
The UNION set operator is used for combining data from two tables which have columns with the same data type. When a UNION is performed the data from both tables will be collected in a single column having the same data type.
Difference between Join and Union
Only primary key & foreign key column & data types of both tables must be same
All column & data types of both tables must be same
Selects columns from 2 or more tables
Selects rows from two or more
Generally used to combine rows from multiple tables
Generally used to merge two or more tables
SELECT a.Column1, b.Column2 FROM TableA a INNER JOIN TableB b ON a.id = b.id
SELECT 1 AS [Column1], 2 AS [Column2]
SELECT 3 AS [Column1], 4 AS [Column2]