为什么此SQL查询返回零行？

Question

I have three tables, menu_level1 , menu_level2 and menu_level3 .

The first table has three columns:

id, url, menuname

I'm hungarian, our language use accented characters, so I have to make menuname column for accented character menus.

The second and the third tables have these columns:

id, url, menuname, relation 

Relation is necessary, because it shows the relationship among the tables. So, I have a menu system for example:

services -> webpage making -> business webpages

the rows looks like below:

menu_level1

url->services

menu_level2

url->webpage making
relation->services

menu_level3

url->business webpages
relation->webpage making

I'd like to select from tables with inner join. I use the following code:

SELECT 
    menu_leve2.url AS url2, menu_level1.url AS url1 
FROM 
    menu_level2 
INNER JOIN 
    menu_level1 ON 'menu_level2.relation' = 'menu_level1.url'

But it returns zero rows. WHY? menu_level2 has rows, where the relation is the same like menu_level2's url.

What is the problem? Why doesn't work this?

Answer 1

You have enclosed the column names in single quotes, turning them into strings rather than references. The two strings are not the same, so the comparison is always false.

Try:

SELECT menu_level2.url AS url2, menu_level1.url AS url1
FROM menu_level2 INNER JOIN
     menu_level1 
    ON menu_level2.relation = menu_level1.url;

If you are learning SQL, I would advise you to use table aliases:

SELECT ml2.url AS url2, ml1.url AS url1
FROM menu_level2 ml2 INNER JOIN
     menu_level1 ml1
    ON ml2.relation = ml1.url;

The resulting queries are easier to write and to read.

It seems strange to me that relation would be equated to url . Without sample data, it is hard to understand whether or not this is correct. But you should check the join conditions to be sure they are what you want.

Answer 2

You are mention the column names in to single quotes.

SELECT 
    menu_leve2.url AS url2, menu_level1.url AS url1 
FROM 
    menu_level2 
INNER JOIN 
    menu_level1 ON menu_level2.relation = menu_level1.url

or you can make the alias of the table and call with the reference of alias.

SELECT 
    menu_leve2.url AS url2, menu_level1.url AS url1 
FROM 
    menu_level2 ml2,
    menu_level1 ml1
WHERE
    ml2.relation = ml1.url