SQL LEFT JOIN 和别名问题

Question

Here is part of my query.

        , dbo.tew_translatedtext.tra_0  as FromParentTagDescription
        , dbo.tew_translatedtext.tra_0  as ToParentTagDescription
    , rrr.tra_0 as ToParentTagDescription

FROM vew_cable_ex  -- as test alias

LEFT JOIN vew_wire_ex  ON ((vew_wire_ex.cab_id = vew_cable_ex.cab_id) AND vew_wire_ex.caw_no = vew_cable_ex.caw_no)

    -- get the parent tag description
    left join dbo.tew_translatedtext   on ( vew_wire_ex.vwircomcomfrom_com_id = dbo.tew_translatedtext.tra_objectid )
    left join dbo.tew_translatedtext  as rrr on ( vew_wire_ex.vwircomcomto_com_id = dbo.tew_translatedtext.tra_objectid )

I have few questions here. - is the 'as rrr' used properly? - The script could run. But seems 'rrr.tra_0 as ToParentTagDescription' didn't give the expected result. all empty. i suspect that the SQL syntax somewhere is incorrect.

Any comment is appreciated.

best Roland

Answer 1

The best practice is to assign all of your tables meaningful aliases, then reference those aliases every time you identify a column in your query. That will help make it clear, both as you write, and when you or someone else has to come back and troubleshoot, which table each column comes from. It also makes the code easier to read.

Once you've aliased the table, many SQL dialects require that you then use the alias in the JOIN...ON criteria.

I should note that in your code snippet, you have the column ToParentTagDescription defined twice, so I repeated that below. Are you sure you're looking at the right one for your expected results, though? You're also calling the column tra_0 from your first join to tew_translatedtext twice and assigning different column aliases to it.

As a matter of readability, I find parenthesis in joins to be visual clutter unless they're necessary for actual grouping of some sort, so I removed them.

...
   fid.tra_0 AS FromParentTagDescription
  ,fid.tra_0 AS ToParentTagDescription
  ,tid.tra_0 AS ToParentTagDescription
FROM 
  <AddSchemaHere>.vew_cable_ex AS cx
LEFT JOIN 
  <AddSchemaHere>.vew_wire_ex AS wx
  ON 
    wx.cab_id = cx.cab_id
    AND 
    wx.caw_no = cx.caw_no
-- get the parent tag description
LEFT JOIN 
  dbo.tew_translatedtext AS fid --From ID...
    ON wx.vwircomcomfrom_com_id = fid.tra_objectid
LEFT JOIN 
  dbo.tew_translatedtext AS tid --To ID 
    ON wx.vwircomcomto_com_id = tid.tra_objectid;

Answer 2

The problem is that you're joining dbo.tew_translatedtext to your query twice, but you're not assigning aliases to each of them and you're not using the alias you have consistently. This means that the database engine you're using (I suspect it's SQL Server, but it would help if you would tag the database engine properly) is guessing about which of the table joins you meant - and it's probably getting it wrong. I suspect that you meant something like:

  .
  .
  .
  , fp.tra_0 as FromParentTagDescription
  , tp.tra_0 as ToParentTagDescription
FROM vew_cable_ex as cx
LEFT JOIN vew_wire_ex as vx
  ON (vx.cab_id = cx.cab_id AND
      vx.caw_no = cx.caw_no)
left join dbo.tew_translatedtext as fp -- get the parent tag description
  on (vx.vwircomcomfrom_com_id = fp.tra_objectid )
left join dbo.tew_translatedtext as tp
  on (vx.vwircomcomto_com_id = tp.tra_objectid )

Here I've used fp as an alias for "from parent", and tp as an alias to "to parent". This seems to be in keeping with the column aliases assigned earlier.

Answer 3

enter image description here

The last issue was fixed.

I'm trying to join another view as the screenshot shows. left join vew_cable_ex as cex on cex.fun_bun_id = vew_bundle.bun_defaultfunid

it throw a error, saying can't bound. it seems it is related to vew_cable_ex table. Could you point the issue out?

Regars, Roland

Answer 4

some notes to my answer: in the ON clause on the table where you alias to rrr, you wrongly aliased the column it looks like.

  , dbo.tew_translatedtext.tra_0  as FromParentTagDescription
    , dbo.tew_translatedtext.tra_0  as ToParentTagDescription
, rrr.tra_0 as ToParentTagDescription

FROM vew_cable_ex  -- as test alias

 LEFT JOIN vew_wire_ex  ON ((vew_wire_ex.cab_id = vew_cable_ex.cab_id) AND 
 vew_wire_ex.caw_no = vew_cable_ex.caw_no)

-- get the parent tag description
left join dbo.tew_translatedtext   on ( vew_wire_ex.vwircomcomfrom_com_id = dbo.tew_translatedtext.tra_objectid )
left join dbo.tew_translatedtext  as rrr on ( vew_wire_ex.vwircomcomto_com_id = rrr.tra_objectid )