Postgres-引用完全限定的表或带标签的表联接后面的列

Question

What's the correct method or syntax for referencing a fully qualified table/column when a group of table joins has been labeled? Context:

SELECT ... FROM 
(A JOIN B ON A.?=B.? JOIN C on B.?=C.?) AS ABCs 
LEFT OUTER JOIN 
(X JOIN W on X.?=W.? JOIN Z on W.?=Z.?) AS XYZs on ABCs.?=XYZ.?

Seemingly when labeling a table, or group of tables you can no longer reference obfuscated tables directly - you must use the label name. For example, in the above code each table has an ID column, which you'd normally reference via A.id or B.id, however when labeled the tables you must reference via ABCs.id - though, this obviously causes the '... is ambiguous' error.

I was hoping for a solution along similar lines to "ABCs.A.id".

FWIW, in practice I'm using this to join a large composite table on to itself, hence the naming convention.

Answer 1

In your query, the FROM clause is evaluated first. That has the effect of hiding table "a" from the rest of the query. (The same rule applies to column aliases, too.)

I threw together some nonsense tables for demonstration. This query works.

SELECT *
FROM 
(A JOIN B ON A.a_id =B.b_id 
   JOIN C on B.b_id=C.c_id) AS ABCs 
LEFT OUTER JOIN 
(X JOIN W on X.x_id=W.w_id 
   JOIN Z on W.w_id=Z.z_id) AS XYZs on ABCs.a_id=XYZs.w_id

But this one, which refers to A.a_id in the SELECT clause, does not.

SELECT A.a_id, *
FROM 
(A JOIN B ON A.a_id =B.b_id 
   JOIN C on B.b_id=C.c_id) AS ABCs 
LEFT OUTER JOIN 
(X JOIN W on X.x_id=W.w_id 
   JOIN Z on W.w_id=Z.z_id) AS XYZs on ABCs.a_id=XYZs.w_id

ERROR:  invalid reference to FROM-clause entry for table "a"
LINE 1: SELECT A.a_id, *
               ^
HINT:  There is an entry for table "a", but it cannot be referenced from 
this part of the query.

Refer to it by the alias you gave to the join expression instead.

SELECT ABCs.a_id, *
FROM 
(A JOIN B ON A.a_id =B.b_id 
   JOIN C on B.b_id=C.c_id) AS ABCs 
LEFT OUTER JOIN 
(X JOIN W on X.x_id=W.w_id 
   JOIN Z on W.w_id=Z.z_id) AS XYZs on ABCs.a_id=XYZs.w_id

To use the same join expression twice, use two different aliases.

SELECT *
FROM 
(A JOIN B ON A.a_id =B.b_id 
   JOIN C on B.b_id=C.c_id) AS ABC_1s 
LEFT OUTER JOIN 
(A JOIN B ON A.a_id =B.b_id 
   JOIN C on B.b_id=C.c_id) AS ABC_2s on ABC_1s.a_id=ABC_2s.c_id

Use the same syntax, alias_name.column_name, in the SELECT clause if you need to.

SELECT ABC_1s.a_id, ABC_2s.a_id, *
FROM 
(A JOIN B ON A.a_id =B.b_id 
   JOIN C on B.b_id=C.c_id) AS ABC_1s 
LEFT OUTER JOIN 
(A JOIN B ON A.a_id =B.b_id 
   JOIN C on B.b_id=C.c_id) AS ABC_2s on ABC_1s.a_id=ABC_2s.c_id