SQL查询以查找其中1列的值与另一行的另一列匹配的行

Question

I have a database table with a column called 'symbol', that is unique via a non-clustered index. We now need to change the data in the 'symbol' column, using the data from another column in the same table, say column2.

Trying to do an update, eg

update table
set symbol = column2
where column2 <> '' and
deleted = 0

results in a 'Cannot insert duplicate key row in object' error, so there must be 1 or more rows existing in the table that already have a value in the symbol column that is equal to the value in column2, or there are some rows that have a duplicate column 2 value.

I can find the rows that have duplicates in column2, but I'm struggling to come up with a query to find those rows that have a value in the symbol column that exists in any row in column2. Any one got any ideas?

Thanks.

Answer 1

select t1.symbol, count(0) as rows
from table t1
join table t2 on t2.column2 = t1.symbol
group by t1.symbol

Answer 2

Test data:

symbol      column2
----------- -----------
1           1
2           1
3           3
4           5

find those rows that have a value in the symbol column that exists in any row in column2.

select symbol, column2
from table 
where symbol in (select column2
                 from table)

Result:

symbol      column2
----------- -----------
1           1
3           3

Or possibly this depending on what result you want.

select symbol, column2
from table as T1
where exists (select *
              from table as T2
              where T1.symbol = T2.column2 and
                    T1.symbol <> T2.symbol)

Result:

symbol      column2
----------- -----------
1           1