如何在 SQL 中加入这 3 个表？

Question

I'm looking for the right SQL request. I'm using SQLite.

I have one principal table "cmdb":

name    | env  | type
-------------------------
VMV1    | Prod | Virtual
SP      | Dev  | Physical
VMO1    | Dev  | Virtual

I have another table "vmv":

name  |    dns     | host  | management
---------------------------------------
VMV1  | VMV1.local | Host1 | Man1
VMV2  | VMV2.local | Host1 | Man1

And a another table "vmo":

name |    dns      | host  | management
---------------------------------------
VMO1  | VMO1.local | Host2 | Man2
VMO2  | VMO2.local | Host2 | Man2

Here is what I would like to get:

 name  | management |    dns     | env  | type
-------------------------------------------------
 VMV1  |    Man1    | VMV1.local | Prod | Virtual
 VMV2  |    Man1    | VMV2.local |      | 
 SP    |            |            | Dev  | Physical
 VMO1  |    Man2    | VMO1.local | Dev  | Virtual
 VMO2  |    Man2    | VMO2.local |      | 

but I cannot manage to get the right result...

Here is what got me the closest to the result. You can see that I'm not really good at all in making SQL requests more complex than "select * from table1"... I tried a lot of combinations and studied lots of examples on stackoverflow and others websites about UNION, JOIN and other SQL keywords but I unfortunetly still don't get it. That's why I'm looking now for help here.

SELECT
c.name, 
v.management, 
v.dns,  
c.env,
c.type 
FROM cmdb AS c
LEFT JOIN vmv AS v ON v.name = c.name
LEFT JOIN vmo AS o ON o.name = c.name

My result is:

 name | management  |    dns     | env  | type
-----------------------------------------------
 VMV1 |    Man1     | VMV1.local | Prod | Virtual
 SP   |             |            | Dev  | Physical
 VMO1 |             | VMO1.local | Dev  | Virtual

Answer 1

You can join with LEFT joins all the distinct names of the 3 tables with each of the tables:

select t.name,
       coalesce(v.management, o.management) management,
       coalesce(v.dns, o.dns) dns,
       c.env,
       c.type
from (
  select name from cmdb union
  select name from vmv union
  select name from vmo
) t
left join cmdb c on c.name = t.name
left join vmv v on v.name = t.name
left join vmo o on o.name = t.name

See the demo .

Or:

select t.name,
       n.management,
       n.dns,
       c.env,
       c.type
from (
  select name from cmdb union
  select name from vmv union
  select name from vmo
) t
left join cmdb c on c.name = t.name
left join (
  select name, dns, management from vmv
  union all
  select name, dns, management from vmo
) n on n.name = t.name

See the demo .

Results:

| name | management | dns        | env  | type     |
| ---- | ---------- | ---------- | ---- | -------- |
| SP   |            |            | Dev  | Physical |
| VMO1 | Man2       | VMO1.local | Dev  | Virtual  |
| VMO2 | Man2       | VMO2.local |      |          |
| VMV1 | Man1       | VMV1.local | Prod | Virtual  |
| VMV2 | Man1       | VMV2.local |      |          |

Answer 2

In principle it looks like you want to do a UNION of vmo and vmv , then do a FULL OUTER JOIN of that result with cmdb . I am not familiar with sqlite so I don't know if the exact syntax might be different from what I would write for Oracle (or if it even supports these operations).

Here's what I would try:

SELECT NVL(cmdb.name, vm_all.name), management, dns, env, type
 FROM
  cmdb
  FULL OUTER JOIN (
    SELECT name, management, dns FROM vmv
    UNION ALL
    SELECT name, management, dns FROM vmo
  ) vm_all
  ON vm_all.name = cmdb.name