SQL查询：我需要两个查询，还是可以使用嵌套子查询

This question may have been asked before but I don't really know what verbiage to search with.

I have a mysql DB that has a table with 3 columns [ID, fieldName and fieldValue] that is used to describe attributes of objects in another table. The ID field stores the foreign key of object in the other table and the fieldName and fieldValue store things like title, description, file size and summary.

I am trying to write a query that returns rows where a fieldName and fieldValue pair match known values and the returned row ID has a another distinct fieldValue in another row. Right now I am accomplishing it with two queries and an if statement. Here is the sudo code:

$result = SELECT * FROM table_a WHERE fieldName = 'title' and fieldValue = 'someTitle'
$test = SELECT * FROM table_a WHERE fieldValue = 'someValue' and id = '{$result['id']}'
if ($test) {
  /* Result Found */
}

You can self-join the table:

SELECT * FROM table_a AS s1 
JOIN table_a AS s2 USING (id)
WHERE
    s1.fieldName = 'Title' AND s1.fieldValue = 'someTitle'
    AND s2.fieldValue = 'someValue'

What you said translated in sql would be:

SELECT b.* 
FROM table_a a
INNER JOIN table_a b ON a.id = b.id
WHERE a.fieldName = 'title'
  AND a.fieldValue = 'someTitle'
  AND a.fieldValue <> b.fieldValue

This gets you the rows in table_a that have the same id as the row with you predefined values, but with a different fieldValue. This assumes that id is not the primary key, otherwise there will not be another row with the same id, but it looks in your question that this isn't the case. (If you want to check for a specific value you can do: AND b.fieldValue = 'someValue' in the last line)