PostgreSQL query from multiple tables - postgresql

Question

Question about querying PostgreSQL.
It's simple to do by stored procedure or inside any programming language, but my question is it possible to do by one select statement. This is an example of two tables.
Thank you.

Answer 1

You would normally do the "group headers" in some kind of reporting tool. However, it can all be done in pure SQL, if you wish.
You start with the data in a "standard JOIN":
-- The basic query
SELECT
table_1.table_1_id, phone, name, some_data,
row_number() over(partition by table_1.table_1_id order by some_data)
FROM
table_1
JOIN table_2 ON table_2.table_1_id = table_1.table_1_id ;
That will produce the following table:
table_1_id | phone | name | some_data | row_number
---------: | :---- | :--- | :-------- | ---------:
1 | 502 | aa | a | 1
1 | 502 | aa | b | 2
1 | 502 | aa | j | 3
1 | 502 | aa | n | 4
2 | 268 | bb | a | 1
2 | 268 | bb | s | 2
2 | 268 | bb | y | 3
5 | 984 | ee | a | 1
5 | 984 | ee | n | 2
5 | 984 | ee | w | 3
If you want to add some header rows and also not show the values of phone, name, we need to *add* these data to the query. This is done by making a second specificSELECTfor the headers, andUNION ALL` with the first one.
That is, we will use:
-- This query produces the 'headers'
SELECT DISTINCT
table_1.table_1_id,
phone,
name,
'' AS some_data,
true as is_header -- this marks this row as a 'header'
FROM
table_1
JOIN table_2 ON table_2.table_1_id = table_1.table_1_id
UNION ALL
-- This query produces the actual data
SELECT
table_1.table_1_id,
phone,
name,
some_data,
false as is_header
FROM
table_1
JOIN table_2 ON table_2.table_1_id = table_1.table_1_id ;
Now, we make this a subquery, and make some extra logic to decide which data needs to be shown, and which one needs to be hidden (actually, shown as ''):
-- A few tricks to do the formatting
SELECT
-- Here we decide which information to show, which not
case when is_header then cast(table_1_id as text) else '' end AS table_1_id,
case when is_header then phone else '' end AS phone,
case when is_header then name else '' end as name,
case when is_header then '' else some_data end as some_data
FROM
(-- This query produces the 'headers'
SELECT DISTINCT
table_1.table_1_id,
phone,
name,
'' AS some_data,
true as is_header -- this marks this row as a 'header
FROM
table_1
JOIN table_2 ON table_2.table_1_id = table_1.table_1_id
UNION ALL
-- This query produces the actual data
SELECT
table_1.table_1_id,
phone,
name,
some_data,
false as is_header
FROM
table_1
JOIN table_2 ON table_2.table_1_id = table_1.table_1_id
) AS q
ORDER BY
q.table_1_id, is_header DESC /* Header goes first */, some_data ;
This produces:
table_1_id | phone | name | some_data
:--------- | :---- | :--- | :--------
1 | 502 | aa |
| | | a
| | | b
| | | j
| | | n
2 | 268 | bb |
| | | a
| | | s
| | | y
5 | 984 | ee |
| | | a
| | | n
| | | w
You can see the whole setup and the simulated data at dbfiddle here
Note that the order that you specified is not preserved. You should have some logic on how things should get ordered. SQL doesn't have a concept of "insert order" or "natural order"; you always have to choose which one you want (otherwise, the database will choose the one it is most convenient for it, and that may change from one execution to the next).

Answer 2

There is the nice feature like grouping sets.
Using it you could to get in single query/result set several sets for different grouping conditions. Thanks for example data to joanolo
Lets start at the most simple query:
SELECT
table_1.table_1_id, phone, name, some_data
FROM
table_1 JOIN table_2 ON table_2.table_1_id = table_1.table_1_id;
┌────────────┬───────┬──────┬───────────┐
│ table_1_id │ phone │ name │ some_data │
╞════════════╪═══════╪══════╪═══════════╡
│ 1 │ 502 │ aa │ a │
│ 1 │ 502 │ aa │ b │
│ 1 │ 502 │ aa │ n │
│ 1 │ 502 │ aa │ j │
│ 5 │ 984 │ ee │ w │
│ 5 │ 984 │ ee │ a │
│ 5 │ 984 │ ee │ n │
│ 2 │ 268 │ bb │ s │
│ 2 │ 268 │ bb │ a │
│ 2 │ 268 │ bb │ y │
└────────────┴───────┴──────┴───────────┘
Here wee need to add the groups by first three columns:
SELECT
table_1.table_1_id, phone, name--, some_data
FROM
table_1 JOIN table_2 ON table_2.table_1_id = table_1.table_1_id
GROUP BY GROUPING SETS ((table_1.table_1_id, phone, name));
┌────────────┬───────┬──────┐
│ table_1_id │ phone │ name │
╞════════════╪═══════╪══════╡
│ 2 │ 268 │ bb │
│ 5 │ 984 │ ee │
│ 1 │ 502 │ aa │
└────────────┴───────┴──────┘
Note double parenthesizes. Actually it is equal to the simple GROUP BY table_1.table_1_id, phone, name (that's because I commented out some_data column, it is not in the group). But we wants to add to our query more data:
SELECT
table_1.table_1_id, phone, name, some_data
FROM
table_1 JOIN table_2 ON table_2.table_1_id = table_1.table_1_id
GROUP By GROUPING SETS ((table_1.table_1_id, phone, name),(table_1.table_1_id, phone, name, some_data));
┌────────────┬───────┬──────┬───────────┐
│ table_1_id │ phone │ name │ some_data │
╞════════════╪═══════╪══════╪═══════════╡
│ 1 │ 502 │ aa │ a │
│ 1 │ 502 │ aa │ b │
│ 1 │ 502 │ aa │ j │
│ 1 │ 502 │ aa │ n │
│ 1 │ 502 │ aa │ ░░░░ │
│ 2 │ 268 │ bb │ a │
│ 2 │ 268 │ bb │ s │
│ 2 │ 268 │ bb │ y │
│ 2 │ 268 │ bb │ ░░░░ │
│ 5 │ 984 │ ee │ a │
│ 5 │ 984 │ ee │ n │
│ 5 │ 984 │ ee │ w │
│ 5 │ 984 │ ee │ ░░░░ │
└────────────┴───────┴──────┴───────────┘
It is almost what we wants. We need just to order the data in proper way and clean some columns:
SELECT
case when some_data is null then table_1.table_1_id end as table_1_id,
case when some_data is null then phone end as phone,
case when some_data is null then name end as name,
some_data
FROM
table_1
JOIN table_2 ON table_2.table_1_id = table_1.table_1_id
group by grouping sets((table_1.table_1_id, phone, name), (table_1.table_1_id, phone, name, some_data))
order by table_1.table_1_id, some_data nulls first;
┌────────────┬───────┬──────┬───────────┐
│ table_1_id │ phone │ name │ some_data │
╞════════════╪═══════╪══════╪═══════════╡
│ 1 │ 502 │ aa │ ░░░░ │
│ ░░░░ │ ░░░░ │ ░░░░ │ a │
│ ░░░░ │ ░░░░ │ ░░░░ │ b │
│ ░░░░ │ ░░░░ │ ░░░░ │ j │
│ ░░░░ │ ░░░░ │ ░░░░ │ n │
│ 2 │ 268 │ bb │ ░░░░ │
│ ░░░░ │ ░░░░ │ ░░░░ │ a │
│ ░░░░ │ ░░░░ │ ░░░░ │ s │
│ ░░░░ │ ░░░░ │ ░░░░ │ y │
│ 5 │ 984 │ ee │ ░░░░ │
│ ░░░░ │ ░░░░ │ ░░░░ │ a │
│ ░░░░ │ ░░░░ │ ░░░░ │ n │
│ ░░░░ │ ░░░░ │ ░░░░ │ w │
└────────────┴───────┴──────┴───────────┘
Bingo! Hope it is what you asking for.