When you work with SQL database you usually need more than 1 table. Some values are in one table and the others are in a different one = division of the information into many tables which are connected between each other (via keys) is the principle of relational database.
There are 5 basic kinds of joins: INNER JOIN, LEFT JOIN (this article), RIGHT JOIN, CROSS JOIN, FULL JOIN
Hint: Commands LEFT OUTER JOIN and LEFT JOIN are equivalent
Now or later you will need to know how to connect the tables. Let’s learn it!
LEFT OUTER JOIN Syntax
SELECT [Table_A].[Column 1] ,[Table_A].[Column 2] ,[Table_B].[Column 3]FROM [Table_A] LEFT OUTER JOIN [Table_B] ON [Table_A].[Id] = [Table_B].[Id]
imagine 2 tables. Each of the tables will consist of only 1 column [Number]:
- Table A consists of column [Number]: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
- Table B consists of column [Number]: 1, 2, 3, 100, 150
What will be the result of the script?
SELECT
[Table_A].[Number], [Table_B].[Number]
FROM [Table_A]
LEFT JOIN [Table_B]
ON [Table_A].[Number] = [Table_B].[Number];
The result is 10 rows:
Hint: Take a look at the examples of all the possible kinds of connections of the tables to better understand the differences between each kind. I use football teams as an example in all of them.
LEFT JOIN example with football teams
We will take a look at the database table joins using football teams. We have 2 tables, take a look at them (sorry, texts in screenshots and object names are in Czech):
- [Fotbalove_tymy] ([ID], [Tym], [Id_mesto]) – table with list of the football teams
- [Mesta] ([ID], [Nazev_mesta]) – table with towns
We will display the list of the football teams with their hometown. Script will look like this:
SELECT [Fotbalove_tymy].[ID] ,[Fotbalove_tymy].[Tym] ,[Mesto].[Mesto]FROM [Fotbalove_tymy] LEFT JOIN [Mesto] ON [Fotbalove_tymy].[Id_Mesto] = [Mesto.Id];
We create a relation between the tables. The requirement by which both records from the table will get connected lies in ON clause.
Apart from INNER JOIN here we have all football teams mentioned in one table as a result. Brno team is shown (left table) but since there is no record of it in table of towns [město_id] = 0 the value [mesto] = NULL – take a look at the difference between this solution and Inner join solution.
