How to Work with Inner Joins
Many times you will need data from more than one table to display on your report. To fetch data from two or more relational tables, you will code a join. The inner join syntax is preferable when you code the join as compared to the older technique of listing the table names without an explicit reference to a join operation.
To learn how work with inner joins, follow these steps:
- You'll need to setup the MySQL database tables. The instructions for the setup can be found in the topic How to add comments in simple SQL selects. Follow steps 1 through 7 before proceeding to the next step.
- Now let's write an inner join.
- Imagine we are required to display city name and the name of the state. The Cities table contains the city name but does not store the state name.
The state name is located on the States table. Therefore, we need to join the Cities table to the States table in order to meet the requirement.
Execute the following statement:
Notice that I've included two tables on the from clause by using the
inner joinkeywords. In addition, I've provided a table alias for each table (e.g., "c" for the Cities table) that I use to qualify potentially ambiguous references (e.g.,
namethat appears in both tables). I use the alias in the select list of columns and in the
onclause. Furthermore, note that in the
oncondition I check to make sure the state abbreviation in the Cities table is equal to the state abbreviation in the States table. This is necessary so that I display the correct state name for that city. If we omit the
oncondition then each city would be joined to each state, producing a much larger (and incorrect) result!