MySQL - Left joins without duplicate columns - mysql

Question

I have an issue with some of the join statements I'm trying to use. I have two tables that need to be joined, with both featuring all of their information. They're as follows.
INSTITUTION
IName | ALocation_ID | IPicture
ADDRESS
ALocation_ID | AStreet | AZip | ...(other relevant fields)
I've been trying to use:
CREATE VIEW InstitutionView
AS SELECT * FROM INSTITUTION
LEFT JOIN ADDRESS
ON INSTITUTION.ALocation_ID=ADDRESS.ALocation_ID;
but the error I receive says something about duplicate columns. What am I doing wrong?

Answer 1

You will have to select the columns individually. Hopefully this helps you out a little.
CREATE VIEW InstitutionView
AS
SELECT address.id,address.iname,address.alocation_id,ipicture,institution.astreet,institution.azip
FROM INSTITUTION
LEFT JOIN ADDRESS
ON INSTITUTION.ALocation_ID=ADDRESS.ALocation_ID;

Answer 2

That is because ALocation_ID column is present in both the tables.
Try creating the view explicitly naming the required columns.
CREATE VIEW InstitutionView
AS SELECT Iname,INSTITUTION.ALocation_ID,IPicture,AStreet,AZip ...
FROM INSTITUTION
LEFT JOIN ADDRESS
ON INSTITUTION.ALocation_ID=ADDRESS.ALocation_ID;

Answer 3

Conceptually, JOIN first creates an intermediate cross-product where the columns are referenced by a table name or alias dotted by a column name from that table; then ON and WHERE filter out rows that don't match to give a second intermediate result. If a column name appears only in one table then you can leave out the table & dot to refer to the column.
MySQL 5.6 Reference Manual :: 13.2.9 SELECT Syntax
You can refer to a column as col_name, tbl_name.col_name, or db_name.tbl_name.col_name. You need not specify a tbl_name or db_name.tbl_name prefix for a column reference unless the reference would be ambiguous. See Section 9.2.1, “Identifier Qualifiers”, for examples of ambiguity that require the more explicit column reference forms.
MySQL 5.6 Reference Manual :: 9.2.1 Identifier Qualifiers
Suppose that tables t1 and t2 each contain a column c, and you retrieve c in a SELECT statement that uses both t1 and t2. In this case, c is ambiguous because it is not unique among the tables used in the statement. You must qualify it with a table name as t1.c or t2.c to indicate which table you mean.
Hence:
CREATE VIEW InstitutionView
AS SELECT IName,I.ALocation_ID,IPicture,AStreet,AZip,...
FROM INSTITUTION I
LEFT JOIN ADDRESS A
ON I.ALocation_ID=A.ALocation_ID;
You might think that if a JOIN is ON or WHERE "=" then there would be no ambiguity. However:
In the case of INNER JOIN, if there were no implicit conversions then columns compared equal would have the same value; but otherwise different values can compare "=". So you can't use just the column name to identify one value.
Moreover for LEFT JOIN, unmatched rows in the left table are extended by NULLs and are added to give a third intermediate result; so in a row a non-NULL in a column in one table can appear with a NULL in the same column of the other table. So again you can't use just the column name to identify one value.
Moreover there doesn't even need to be a test of equality of two columns in a JOIN, or even mention both of or either of columns with a shared name. So the result can have two columns (one from each input table) sharing a name where there is no expectation of equality.