遍历临时表以在SQL Server 2014中构建字符串的最佳方法

Question

I have created a temp table, the idea being that I want to loop through it, match all records with the same email address and then populate a string, which will go in to an email, then drop the table. This will be run as a stored procedure.

I've used a cursor that first grabbed all the unique email addresses and then coalesce the records but with potentially 100k-500k records performance won't be acceptable, and I know there must be a far more efficient way of doing it.

Example data (apologies, don't know how to format it properly)

#temptable

temp_email, temp_string

test@test.com string1
test@test.com string2
test2@test.com string3
test2@test.com string4
test3@test.com string5

I then want to populate another table with this data

emailto...   emailbody

test@test.com   'string1<br / > string2'
test2@test.com   'string3<br / > string4'
test3@test.com   'string5'

Thank you.

Answer 1

The STUFF and FOR XML PATH method achieves this nicely in SQl Server 2014 and prior. Because you have the characters < and > however, they need to be "un-escaped" afterwards:

WITH VTE AS(
    SELECT *
    FROM (VALUES('test@test.com','string1'),
                ('test@test.com','string2'),
                ('test2@test.com','string3'),
                ('test2@test.com','string4'),
                ('test3@test.com','string5')) V(Email, String))
SELECT Email,
       STUFF(REPLACE(REPLACE((SELECT '<br/>' + sq.String
                              FROM VTE sq
                              WHERE sq.Email = V.Email
                              FOR XML PATH('')),'&lt;','<'),'&gt;','>'),1,5,'')
FROM VTE V
GROUP BY Email;

Answer 2

you dont need to use cursor, please use string_agg function.

Create table #temptable 
(temp_email varchar(50), temp_string varchar(50))

INSERT INTO #temptable
VALUES ('test@test.com', 'string1'),
('test@test.com', 'string2'),
('test2@test.com', 'string3'),
('test2@test.com', 'string4'),
('test3@test.com', 'string5')


Select temp_email, STRING_AGG(temp_string,' <br/>')
from #temptable
Group by temp_email

Answer 3

In SQL Server 2014 there are ways of doing this without a cursor but they are basically quite convoluted hacks and lead to pretty unreadable SQL in my opinion. See here for details:

How to concatenate text from multiple rows into a single text string in SQL server?

A cursor is arguably the best way in SQL 2014 because at least its readable.

In Sql Server 2017 there is an official aggregation function for this:

String_Agg

... but thats no use to you at the mo. Sorry.

Answer 4

You can do something like this :

-- Create temp table
CREATE TABLE #temptable (temp_email varchar(50), temp_string varchar(50))

-- populate table with data
INSERT INTO #temptable
VALUES ('test@test.com', 'string1'),
('test@test.com', 'string2'),
('test2@test.com', 'string3'),
('test2@test.com', 'string4'),
('test3@test.com', 'string5')

-- actual query
    ;WITH CTE_table AS(
     SELECT C.temp_email,
      REPLACE(REPLACE(STUFF(
        (SELECT  CAST(temp_string AS VARCHAR(20))+'<br/>' AS [text()]
         FROM #temptable AS O
         WHERE C.temp_email= o.temp_email
         FOR XML PATH('')), 1, 0, NULL)
         ,'&lt;','<') -- replace this < with html code &lt;
         ,'&gt;','>') -- replace this > with html code &gt;
         AS temp_string
         ,ROW_NUMBER() OVER (partition by temp_email order by temp_email) rownumber
    FROM #temptable AS C
    ) 
    -- Get only unique records
    SELECT temp_email,temp_string FROM CTE_table 
    Where rownumber=1