不使用任何主键时如何联接两个表？

Question

I'm trying to JOIN 2 tables using columns that aren't either of the tables respective Primary Keys:

SELECT *
FROM TableA A
INNER JOIN TableB B
ON A.col5 = B.col5

Yet the above is returning 0 results even though I know for sure that there are rows in Table A whose col5 values match values in col5 of Table B .

What am I doing wrong?

Answer 1

You query:

SELECT *
FROM TableA A
INNER JOIN TableB B
ON A.col5 = B.col5;

Has the correct syntax for a join. If there are matching values, then it will return it. (Or course, you could be calling this from an application and there could be errors either in the application code or the connection to the database, but that is another issue.)

Some cases where values look the same but are not:

• Both values are floats. They look the same when printed out. But the bit at the end of the value differs. NEVER USE FLOATs FOR EQUI-JOINs.
• One value is a number and the other value is a string. The conversion of one of the values results in a slightly different value.
• One value is a date/time and the other is a string. The conversion of one of the values results in a slightly different value.
• The values are strings. They differ in case. You think abc is the same as ABC1 , but SQL doesn't.
• You have spaces or other "hidden" characters at the end of the string.

And two more reasons that I can think of but may not be true in all databases:

• One value is a char and the other varchar (or wide versions of them). The implicit spaces at the end of the char make them different.
• One is a unicode string and the other ASCII string. Two characters look the same but are not.

Answer 2

Your query is correct and should return the matching values in the table. But as you said there is invisible space in the values in other table at the end try using LTRIM and RTRIM to remove the invisible space from the values. LTRIM removes the whitespaces from the beginning of the string and RTRIM removes the whitespaces from the end of the string.

The query would be like :

SELECT * FROM TableA A INNER JOIN TableB B ON A.col5 = LTRIM(RTRIM(B.col5))

I hope it helps you out.