SQL查找相同的列数据

Question

I have a table with around 15 columns. What I would like to be able to do, is select a range of IDs and have all column data that is the same, presented to me.

At the minute, I have it structured as the following:

SELECT id, col_a, col_b ... count(id)
FROM table
GROUP BY col_a, col_b ... 

Which returns rows grouped together that have identical data within all the rows - which is half what I want, but ideally I would like to be able to get a single row with either the value (if it's the same for every row id) or NULL if there is a single difference.

I'm not sure that it is possible, but I would rather see if it's doable in an SQL query than write some looping logic for PHP to go through and check each row's similarity.

Thanks,

Dan

UPDATE:

Just to keep this up-to-date, I worked through the problem by writing a PHP function that would find which were duplicates and then display the differences. However I have now since made a table for each column, and made the columns as references to the other tables.

E.G. In MainTable, ColA now refers to the table ColA

I'm still solving the problem with the PHP for the time being, mainly as I think it still leaves the problem mentioned above, but at least now Im not storing duplicate information.

Answer 1

Its a hairy thing to do, but you could do it similarly to how David Martensson suggested, I would write it like this, however:

Select a.id, a.col1, a.col2, a.col3
FROM myTable a, myTable b
WHERE a.id != b.id
and a.col1 = b.col1
and a.col2 = b.col2
and a.col3 = b.col3

That would give you the ids that are unique, but each result would have the same values for columns 1, 2, and 3. However, I agree with some of the commenters to your question that you should consider an alternative data structure, as this could better take advantage of an RDBMS model. In that case you would want to have 2 tables:

Table name: MyTableIds Fields: id, attrId

Table name: MyTableAttrs Fields: attrId, attr1, attr2, attr3, ect

In general, if you have data that is going to be duplicated for multiple records, you should pull it into a second table and create a relationship so that you only have to store the duplicated data 1 time and then reference it multiple times.