The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type.
One of my work colleagues at work showed me something that I never knew about UNION SQL statement. By default the UNION statement does the exact same thing as a SELECT DISTINCT on the resulting data. But what if we want to carry out a UNION without distincting the data result? Thats where the UNION ALL comes into play.
Example
Table 1: tblCars-UK
| Car_ID | Car_Name |
| 1 | Volvo |
| 2 | Volkswagen |
| 3 | Chevrolet |
| 4 | Nissan |
| 5 | BMW |
Table 2: tblCars-US
| Car_ID | Car_Name |
| 1 | Pontiac |
| 2 | Chrysler |
| 3 | Chevrolet |
| 4 | Dodge |
| 5 | BMW
|
If we used a UNION statement, the results would be as follows:
Select car_name from tblCars-UK
UNION
Select car_name from tbleCars-US
| Car_Name |
| Volvo |
| Volkswagen |
| Chevrolet |
| Nissan |
| BMW |
| Pontiac |
| Chrysler |
| Dodge |
As you can see from the results above that the duplicate car entries have been removed and only displayed once. Now this is what will happen if we use UNION ALL statement:
Select car_name from tblCars-UK
UNION ALL
Select car_name from tbleCars-US
| Car_Name |
| Volvo |
| Volkswagen |
| Chevrolet |
| Nissan |
| BMW |
| Pontiac |
| Chrysler |
| Chevrolet |
| Dodge |
| BMW |