从T-SQL中的其他列派生列

Question

I have a table in T-SQL in books with just one column so far, which has a bunch of book titles:

    Book Title
------------------
Romeo and Juliet

Hamlet

All The King's Men

What I am trying to do is add a second column, called "Other Titles" that contains a string of all the other titles in the column except for the title in the current row, ie

 Book Title                    Other Titles
------------------          ----------------
Romeo and Juliet            Hamlet, All The Kings Men

Hamlet                      R & J, All The King's Men

All The King's Men          R&J, Hamlet

I know I have to probably use STRING_AGG in T-SQL for this, but I just can't seem to figure it out. Any ideas?

Answer 1

If your sql-server version support STRING_AGG function.

You can try to do self-join by condition t2.[Book Title] <> t1.[Book Title] then use STRING_AGG function,

CREATE TABLE T(
 [Book Title] VARCHAR(50)
);

INSERT INTO T VALUES ('Romeo and Juliet');
INSERT INTO T VALUES ('Hamlet');
INSERT INTO T VALUES ('All The King''s Men');


SELECT t1.[Book Title],
       STRING_AGG ( t2.[Book Title] , ',' ) 'Other Titles'
FROM T t1 
INNER JOIN T t2 ON t2.[Book Title] <> t1.[Book Title]
group by t1.[Book Title]

sqlfiddle

Answer 2

I would do this using a correlated subquery:

select b.*,
       (select string_agg(b2.title, ', ') within group (order by b2.title)
        from books b2
        where b2.title <> b.title
       ) as other_titles
from books b;

A correlated subquery makes it easier to keep other columns that you might have in your table.

To be honest, though, the performance is not going to be particularly good. You might consider aggregating everything together and then removing your title:

select b.*,
       stuff(replace(bb.titles, ', ' + b.title, ''), 1, 2, '') as other_titles
from books b cross join
     (select ', ' + string_agg(b2.title, ', ') as titles
      from books
     ) bb;

Calling replace() on titles would typically have better performance than looping through the entire table to reconstruct the string for each row.