Select columns from various tables in mySQL - mysql

Question

I have a dynamic list of categories, which is retrieved from mySQL. When the user is clicking on a category, it loads images for that specific category from the DB. This is the function that does it:
function getImages($categoryID)
{
return json_encode(select("SELECT * FROM images WHERE categoryID=$categoryID"));
}
Now I also want to retrieve also the category name again from the "categories" table (to include the name of the selected category in other part of the page). I tried to do this:
function getImages($categoryID)
{
return json_encode(select(
"SELECT categories.categoryName, images.imageFileName, images.imageHeader
FROM images JOIN categories
ON categories.categoryID = images.categoryID
WHERE categoryID=$categoryID"));
}
but that didn't work, now I don't even get the images.
How can it be done?

Answer 1

In your WHERE clause, categoryID is ambiguous.
WHERE categoryID=$categoryID
Since you have two tables with categoryID, you need to identify which one the WHERE should be looking at.
WHERE categories.categoryID=$categoryID
To make life easier for yourself, you might want to consider aliasing your tables, like this:
SELECT c.categoryName, i.imageFileName, i.imageHeader
FROM images i
JOIN categories c ON c.categoryID = i.categoryID
WHERE c.categoryID=$categoryID
Also, which language are you developing in? If this is a web app, you might want to consider escaping $categoryID to protect against SQL injection.

Answer 2

There is an error in the SQL query, categoryID is ambiguous. Try:
SELECT
categories.categoryName,
images.imageFileName,
images.imageHeader
FROM
images
JOIN
categories on categories.categoryID = images.categoryID
WHERE
categories.categoryID=$categoryID;

Answer 3

Rehashing what's already been said, try a left join so that images that aren't categorized don't get filtered out and use an escape such as the following:
$query = "SELECT c.categoryName, i.imageFileName, i.imageHeader
FROM images i
LEFT JOIN categories c
ON c.categoryID = i.categoryID
WHERE c.categoryID= " . intval($categoryID) . ";"

Answer 4

I think you must qualify the field in your WHEREclause with a table name, i.e.
WHERE image.categoryID=$categoryID
The databse engine does not know which of the two categoryID fields you mean. If you run into problems like this again, you should test the query on the MySQL command line. There you get an error message, that is very helpful in fixing your error.