根据另一列 Sql 服务器中给出的公式更新表中的列

Question

I have a table with 4 columns:

Col1  Col2      Col3       Col4
1      2      Col1+Col2     3
3      4      Col1*Col2     12
6      3      Col1/Col2     2

Col1, Col2 and would have some values, col3 would have a formula and based on that formula I need to populate values in col4. I have tried it using dynamic SQL which works row by row but I have millions of records in my table and it is taking forever to execute the script. Please suggest the simplest and fastest way to handle this scenario.

Answer 1

You could write an update query for each operator if you only have a single operator in the column +,-,/,*

Depending on the distribution of the data solving the simple operations first in bulk, will speed up the process

-- Bulk operation for Col1 * Col2
UPDATE Table
    Set Col4 = Col1 * Col2
WHERE LEN(Col4) - LEN(REPLACE(Col4, '*', '')) = 1 -- Only has one *
AND Like 'Col1%'
AND Like '%Col2'
-- Doesn't have other operator
AND NOT LIKE '%+%'
AND NOT LIKE '%*%'
AND NOT LIKE '%/%'

You could generate these queries using T4 if you have multiple columns/combinations.

For multiple operators you will need a parser because you need to take care or order of operators and other rules.

Answer 2

This is one of the (very) rare cases where a cursor is appropriate. In fact, this is rather complicated -- an updatable cursor along with dynamic SQL.

Here is the idea:

declare @col1 int;
declare @col2 int;
declare @col3 nvarchar(max);
declare @sql nvarchar(max);
declare @col4 int;


DECLARE c CURSOR
FOR SELECT col1, col2, col3 FROM t FOR UPDATE;

OPEN c;

FETCH NEXT FROM c INTO @col1, @col2, @col3;

WHILE @@fetch_status = 0
BEGIN
    set @sql = 'select @col4=[col3] from (select @col1 as col1, @col2 as col2) t';
    
    set @sql = replace(@sql, '[col3]', @col3);
    
    exec sp_executesql @sql, N'@col1 int, @col2 int, @col4 int output',
                       @col1=@col1, @col2=@col2, @col4=@col4 output;

    SELECT @col1, @col2, @col3, @col4;
    
    UPDATE t
        set col4 = @col4
        WHERE current of c;
    
    FETCH NEXT FROM c INTO @col1, @col2, @col3;
END;

CLOSE c;
DEALLOCATE c;

And here is a db<>fiddle.

Note that this will not be speedy. Getting performance on such a query is quite difficult, but you can break the table up into chunks and update each chunk using a separate thread.

Answer 3

You can change your approach and work in set based mode, using also programming language as C#

With running total query you can calculate the distribution of your data and work with chunk of x records to update by Col3. In order not to have lock escalation I advise to work with 1000 rows at a time. Example for running total query:

https://codingsight.com/calculating-running-total-with-over-clause-and-partition-by-clause-in-sql-server/

Now, you can update Col4 with equal chunk (calculated before) using Col3 as condition in a C# cycle or other programming languages (you can also use a while loop in SQL but it isn't recommended)

PS check your indexes before to run the procedure

Answer 4

If the expressions in col3 are syntactically correct in a way SQL server can understand and execute them, you can use dynamic SQL. Create a query with a CASE expression. In the case expression, every branch checks for the string value of col3 and returns the respective expression. You just need as many branches as there are distinct expressions.

DECLARE @query nvarchar(MAX);

SELECT @query = concat('UPDATE elbat', nchar(13), nchar(10),
                       '       SET col4 = CASE', nchar(13), nchar(10),
                       (SELECT DISTINCT
                               concat('                    WHEN col3 = ''', col3, '''', nchar(13), nchar(10),
                                      '                      THEN ', col3, nchar(13), nchar(10)) [text()]
                               FROM elbat
                               FOR XML PATH(''),
                                       TYPE).value('.', 'nvarchar(max)'),
                       '                  END;')
       WHERE EXISTS (SELECT *
                            FROM elbat);

PRINT @query;

EXEC(@query);

db<>fiddle