sql-query，查找并替换为此特定代码

Question

I need to understand the following SQL query and would like to ask if anybody could explain it to me a bit more in detail (like the xml path) as well as update it with a replace element? So I want to find all values with BlaBlaBlaBla and replace them with HaHaHaHa instead. At the moment the query is finding all values of BlaBlaBlaBla only.

DECLARE @searchstring  NVARCHAR(255)
SET @searchstring = '%BlaBlaBlaBla%'

DECLARE @sql NVARCHAR(max)

SELECT @sql = STUFF((
  SELECT 
    ' UNION ALL SELECT ''' + TABLE_SCHEMA + '.' + TABLE_NAME + ''' AS tbl, ''' + COLUMN_NAME + ''' AS col, [' + COLUMN_NAME + '] AS val' + 
    ' FROM [' + TABLE_SCHEMA + '].[' + TABLE_NAME + '] WHERE [' + COLUMN_NAME + '] LIKE ''' + @searchstring + ''''
    FROM INFORMATION_SCHEMA.COLUMNS 
    WHERE DATA_TYPE in ('nvarchar', 'varchar', 'char', 'ntext') FOR XML PATH('')) ,1, 11, '')

Exec (@sql)

Answer 1

I believe that the XML PATH is a trick to get the strings to all concatenate together.

You could change it to REPLACE with something like this:

SELECT @sql = 
SELECT
    ' UPDATE ' + QUOTENAME(TABLE_SCHEMA) + '.' + QUOTENAME(TABLE_NAME) + '
      SET ' + QUOTENAME(COLUMN_NAME) + ' = REPLACE(' + QUOTENAME(COLUMN_NAME) + ', ''' + @search_string + ''', ' + ', ''' + @replace_string + '''
      WHERE ' + QUOTENAME(COLUMN_NAME) + ' LIKE ''' + @searchstring + ''''
FROM INFORMATION_SCHEMA.COLUMNS 
WHERE
    DATA_TYPE in ('nvarchar', 'varchar', 'char', 'ntext') FOR XML PATH('')

EXEC(@sql)

Some caveats:

I haven't tested this. When you're generating code like this it's very easy to make minor errors with all of the start and end quotes, etc. I would print out the SQL and check it, repeating as necessary until you get the output SQL correct.

Also, this is generally not a good idea. If your database is large and/or has a large number of tables then performance is going to be miserable. You should usually do the analysis of where you think this sort of data is going to appear and write code that will correct it as necessary. The fact that data elements are buried in strings throughout your data is concerning.

Finally, be aware that this might easily update additional data that you didn't intend to update. If you try to update "123" with "456" and there's a string out there that is "My ID is 1234" it's going to become "My ID is 4564".

BTW, the QUOTENAME function is a way of enclosing your table and column names in [ and ] , but if the quote character is changed in a DB implementation it should still work.