表联接的SQL数据库错误

Question

I keep getting an error with a table I'm trying to create from my sql database. I'm supposed to look at the inventory from my tables qsale and qdel, find any department(s) that sold the exact the number of some item that was delivered to that department.

This code I wrote keeps giving me this error:

watson=> select itemname,deptname,saleqty from qsale WHERE NOT EXISTS (select su
m(delqty) as total_delqty from qdel where total_delqty > saleqty) group by itemn
ame,deptname,saleqty order by saleqty DESC;
ERROR:  column "total_delqty" does not exist
LINE 1: ...elect sum(delqty) as total_delqty from qdel where total_delq...

I'm trying to join the tables and find where the delivery quantity equals the sale quantity for items from the qsale table. If the item is delivered to a different department then it is not combined with other items. The result I should get is 3 rows with the following:

Geo positioning system | Books | 1

Sextant | Books | 1

Hat - polar explorer | Clothes | 3

Basically I'm not sure what I'm doing wrong with my code to get these specific results. I don't know why SQL isn't recognizing the column 'total_delqty' I tried creating with my sum of delqty.

These are the tables for my SQL database (psql (9.4.4)):

The sale table:

watson=> select * from qsale;
 saleno | saleqty |           itemname           |  deptname
--------+---------+------------------------------+------------
   1001 |       2 | Boots - snakeproof           | Clothes
   1002 |       1 | Pith helmet                  | Clothes
   1003 |       1 | Sextant                      | Navigation
   1004 |       3 | Hat - polar explorer         | Clothes
   1005 |       5 | Pith helmet                  | Equipment
   1006 |       1 | Pocket knife - Nile          | Clothes
   1007 |       1 | Pocket knife - Nile          | Recreation
   1008 |       1 | Compass                      | Navigation
   1009 |       1 | Geo positioning system       | Navigation
   1010 |       5 | Map measure                  | Navigation
   1011 |       1 | Geo positioning system       | Books
   1012 |       1 | Sextant                      | Books
   1013 |       3 | Pocket knife - Nile          | Books
   1014 |       1 | Pocket knife - Nile          | Navigation
   1015 |       1 | Pocket knife - Nile          | Equipment
   1016 |       1 | Sextant                      | Clothes
   1017 |       1 | Sextant                      | Equipment
   1018 |       1 | Sextant                      | Recreation
   1019 |       1 | Sextant                      | Furniture
   1020 |       1 | Pocket knife - Nile          | Furniture
   1021 |       1 | Exploring in 10 Easy Lessons | Books
   1022 |       1 | How to Win Foreign Friends   | Books
   1023 |       1 | Compass                      | Books
   1024 |       1 | Pith helmet                  | Books
   1025 |       1 | Elephant polo stick          | Recreation
   1026 |       1 | Camel saddle                 | Recreation
(26 rows)

The delivery table (arranged to group items together and sum all the deliveries):

watson=> select itemname,deptname, sum(delqty) as total_delqty from qdel group
by itemname,deptname order by deptname DESC;
           itemname           |  deptname  | total_delqty
------------------------------+------------+--------------
 Pocket knife - Avon          | Recreation |            5
 Pocket knife - Nile          | Recreation |           10
 Pith helmet                  | Recreation |            5
 Sextant                      | Recreation |            5
 How to Win Foreign Friends   | Recreation |            2
 Exploring in 10 Easy Lessons | Recreation |            2
 Tent - 8 person              | Recreation |            2
 Tent - 2 person              | Recreation |            5
 How to Win Foreign Friends   | Navigation |            5
 Sextant                      | Navigation |            5
 Geo positioning system       | Navigation |           11
 Exploring in 10 Easy Lessons | Navigation |            5
 Compass                      | Navigation |           51
 Pocket knife - Nile          | Navigation |           60
 Map case                     | Navigation |           30
 Pith helmet                  | Navigation |            5
 Map measure                  | Navigation |           35
 Pith helmet                  | Furniture  |            5
 Pocket knife - Nile          | Furniture  |           10
 Pith helmet                  | Equipment  |           25
 Compass                      | Equipment  |           25
 Pocket knife - Nile          | Equipment  |           30
 Sextant                      | Equipment  |            2
 Boots - snakeproof           | Equipment  |            2
 Pith helmet                  | Clothes    |           19
 Pocket knife - Nile          | Clothes    |           10
 Hat - polar explorer         | Clothes    |            3
 Stetson                      | Clothes    |            3
 Boots - snakeproof           | Clothes    |            5
 Geo positioning system       | Books      |            1
 How to Win Foreign Friends   | Books      |           10
 Pith helmet                  | Books      |            5
 Pocket knife - Nile          | Books      |           10
 Compass                      | Books      |            5
 Sextant                      | Books      |            1
 Exploring in 10 Easy Lessons | Books      |           10
(36 rows)

Answer 1

In Postgres you cannot use column aliases in WHERE clause. Use HAVING instead (because you gives an aggregate function in a condition):

select itemname, deptname, saleqty 
from qsale 
WHERE NOT EXISTS (
    select sum(delqty) as total_delqty 
    from qdel 
    having sum(delqty) > saleqty) 
group by itemname, deptname, saleqty 
order by saleqty DESC;

Answer 2

It looks like it doesn't recognize this

total_delqty

Because that's an alias for an aggregated function where you sum(delqty).

So, if you created a view of the table on the bottom as it is, your query should work, otherwise you'll have to refer to it differently because you can't use an aggregate function in your where clause and you have to join on the aggregated value.

Answer 3

You cannot use total_delqty in your WHERE clause predicate because it is an alias for the aggregate function. Normally, you can't use an aggregate function in a WHERE Clause, but you can if it is already in your SELECT list (your "projection" in database speak). Instead, try WHERE Sum(delqty) > saleqty , thus:

select itemname,deptname,saleqty
from qsale
WHERE NOT EXISTS
    (select sum(delqty) as total_delqty
     from qdel
     where sum(delqty > saleqty))
group by itemname,deptname,saleqty
order by saleqty DESC;