Getting most used words from a column of strings in SQL

As Blogbeard said, the query you provided does not work with SQL Server. Here is one way to count the most used word. This is based from a function, DelimitedSplitN4K, written by Jeff Moden and improved by members of the SQL Server Central community.

ONLINE DEMO

WITH E1(N) AS (
    SELECT 1 FROM (VALUES
        (1),(1),(1),(1),(1),(1),(1),(1),(1),(1)
    ) t(N)
),
E2(N) AS (SELECT 1 FROM E1 a CROSS JOIN E1 b),
E4(N) AS (SELECT 1 FROM E2 a CROSS JOIN E2 b)
SELECT TOP 50
    x.Item,
    COUNT(*)
FROM Posts p
CROSS APPLY (
    SELECT 
        ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
        Item       = LTRIM(RTRIM(SUBSTRING(p.Title, l.N1, l.L1)))
        FROM (
            SELECT s.N1,
                L1 = ISNULL(NULLIF(CHARINDEX(' ',p.Title,s.N1),0)-s.N1,4000)
            FROM(
                SELECT 1 UNION ALL
                SELECT t.N+1 
                FROM(
                    SELECT TOP (ISNULL(DATALENGTH(p.Title)/2,0))
                        ROW_NUMBER() OVER (ORDER BY (SELECT NULL))
                    FROM E4
                ) t(N)
                WHERE SUBSTRING(p.Title ,t.N,1) = ' '
            ) s(N1)
        ) l(N1, L1)
) x
WHERE x.item <> ''
GROUP BY x.Item
ORDER BY COUNT(*) DESC

Since creation of function is not allowed, I've written it that way. Here is the function definition if you're interested:

CREATE FUNCTION [dbo].[DelimitedSplitN4K](
    @pString NVARCHAR(4000), 
    @pDelimiter NCHAR(1)
)
RETURNS TABLE WITH SCHEMABINDING AS
RETURN

WITH E1(N) AS (
    SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL 
    SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
),
E2(N) AS (SELECT 1 FROM E1 a, E1 b),
E4(N) AS (SELECT 1 FROM E2 a, E2 b),
cteTally(N) AS(
    SELECT TOP (ISNULL(DATALENGTH(@pString)/2,0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
),
cteStart(N1) AS (
    SELECT 1 UNION ALL 
    SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(@pString,t.N,1) = @pDelimiter
),
cteLen(N1,L1) AS(
    SELECT s.N1,
        ISNULL(NULLIF(CHARINDEX(@pDelimiter,@pString,s.N1),0)-s.N1,4000)
    FROM cteStart s
)
SELECT 
    ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
    Item       = SUBSTRING(@pString, l.N1, l.L1)
FROM cteLen l
;

And here is how you would use it:

SELECT TOP 50
    x.Item,
    COUNT(*)
FROM Posts p
CROSS APPLY dbo.DelimitedSplitN4K(p.Title, ' ') x
WHERE LTRIM(RTRIM(x.Item)) <> ''
GROUP BY x.Item
ORDER BY COUNT(*) DESC

The result:

Item             
-------- ------- 
to       3812411 
in       3331522 
a        2543636 
How      1770915 
the      1534298 
with     1341632 
of       1297468 
and      1166664 
on       970554  
from     964449  
for      886007  
not      835979  
is       704724  
using    703007  
I        633838  
-        632441  
an       548450  
when     449169  
file     409717  
how      358745  
data     335271  
do       323854  
can      310298  
get      305922  
or       266317  
error    263563  
use      258408  
value    254392  
it       251254  
my       238902  
function 235832  
by       231025  
Android  228308  
as       216654  
array    209157  
working  207445  
does     207274  
Is       205613  
multiple 203336  
that     197826  
Why      196979  
into     196591  
after    192056  
string   189053  
PHP      187018  
one      182360  
class    179965  
if       179590  
text     174878  
table    169393  

With the now available STRING_SPLIT function (since SQL Server 2016, Compatability Level 130) this query becomes much easier:

SELECT TOP 50
       value [word]
     , COUNT(*) [#times]
FROM posts p
CROSS APPLY STRING_SPLIT(p.title, ' ')
GROUP BY value
ORDER BY COUNT(*) DESC

See it in action on the Stack Exchange Data Explorer where it still runs under 2 minutes for the current number of posts in the Stack Overflow database. On Stack Overflow em Português it runs without having to fear for the dreaded timeout.

Results are similar to what you saw in the answer from Felix:

Query solution (No Split Function Required)

PostgreSQL

select word, count(*) from 
(
    -- get 1st words
    select split_part(title, ' ', 1) as word
    from posts

    union all

    -- get 2nd words
    select split_part(title, ' ', 2) as word
    from posts

    union all

    -- get 3rd words
    select split_part(title, ' ', 3) as word
    from posts

    -- can do this as many times as the number of words in longest title

) words
where word is not null
and word NOT IN ('', 'and', 'for', 'of', 'on')
group by word
order by count desc
limit 50;

for a concise version, see: https://dba.stackexchange.com/a/82456/95929