Update a table using JOIN in SQL Server? – Dev

The best answers to the question “Update a table using JOIN in SQL Server?” in the category Dev.

QUESTION:

I want to update a column in a table making a join on other table e.g.:

UPDATE table1 a 
INNER JOIN table2 b ON a.commonfield = b.[common field] 
SET a.CalculatedColumn= b.[Calculated Column]
WHERE 
    b.[common field]= a.commonfield
AND a.BatchNO = '110'

But it is complaining :

Msg 170, Level 15, State 1, Line 2
Line 2: Incorrect syntax near ‘a’.

What is wrong here?

ANSWER:

Try it like this:

    UPDATE a 
    SET a.CalculatedColumn= b.[Calculated Column]
    FROM table1 a INNER JOIN table2 b ON a.commonfield = b.[common field] 
    WHERE a.BatchNO = '110'

ANSWER:

You don’t quite have SQL Server’s proprietary UPDATE FROM syntax down. Also not sure why you needed to join on the CommonField and also filter on it afterward. Try this:

UPDATE t1
  SET t1.CalculatedColumn = t2.[Calculated Column]
  FROM dbo.Table1 AS t1
  INNER JOIN dbo.Table2 AS t2
  ON t1.CommonField = t2.[Common Field]
  WHERE t1.BatchNo = '110';

If you’re doing something really silly – like constantly trying to set the value of one column to the aggregate of another column (which violates the principle of avoiding storing redundant data), you can use a CTE (common table expression) – see here and here for more details:

;WITH t2 AS
(
  SELECT [key], CalculatedColumn = SUM(some_column)
    FROM dbo.table2
    GROUP BY [key]
)
UPDATE t1
  SET t1.CalculatedColumn = t2.CalculatedColumn
  FROM dbo.table1 AS t1
  INNER JOIN t2
  ON t1.[key] = t2.[key];

The reason this is really silly, is that you’re going to have to re-run this entire update every single time any row in table2 changes. A SUM is something you can always calculate at runtime and, in doing so, never have to worry that the result is stale.

ANSWER:

MERGE table1 T
   USING table2 S
      ON T.CommonField = S."Common Field"
         AND T.BatchNo = '110'
WHEN MATCHED THEN
   UPDATE
      SET CalculatedColumn = S."Calculated Column";

ANSWER:

Answer given above by Aaron is perfect:

UPDATE a
  SET a.CalculatedColumn = b.[Calculated Column]
  FROM Table1 AS a
  INNER JOIN Table2 AS b
  ON a.CommonField = b.[Common Field]
  WHERE a.BatchNo = '110';

Just want to add why this problem occurs in SQL Server when we try to use alias of a table while updating that table, below mention syntax will always give error:

update tableName t 
set t.name="books new" 
where t.id = 1

case can be any if you are updating a single table or updating while using join.

Although above query will work fine in PL/SQL but not in SQL Server.

Correct way to update a table while using table alias in SQL Server is:

update t 
set t.name="books new" 
from tableName t 
where t.id = 1

Hope it will help everybody why error came here.