Using #DECLARE in MYSQL 5.1 - mysql

Question

just playing around with some sql statements on MYSQL Server 5.1
I already asked a question how to make a specific count on two tables (see here)
and I also found an answer how to transpose my result (see here) but I cant use it on my local MYSQL Server 5.1.
This is table one: test
id|name|test_type
-------------
1|FirstUnit|1
2|FirstWeb|2
3|SecondUnit|1
The second table: test_type
id|type
--------
1|UnitTest
2|WebTest
The following result will be written into "yourtable" (a temporary table)
type|amount
-----------
UnitTest|2
WebTest|1
What I want to have at the end is:
UnitTest|WebTest
-----------------
2|1
(The problem is I think, the the last part is from an MS-SQL example, so it will not work on MYSQL)
This are my sql statements:
--create a temporary table
create temporary table IF NOT EXISTS yourtable
(
test_type varchar(255),
amount varchar(255)
);
--make a selecten into the temporary table
INSERT INTO yourtable
SELECT
t.test_type ,
COUNT(*) AS amount
FROM test_types AS t
JOIN test AS te ON t.id= te.test_type
GROUP BY test_type
ORDER BY t.test_type;
--just for debugging
select * from yourtable;
-- transpose result
DECLARE #cols AS NVARCHAR(MAX),
#query AS NVARCHAR(MAX)
select #cols = STUFF((SELECT distinct ',' + QUOTENAME(Type)
from yourtable
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)')
,1,1,'')
set #query = 'SELECT ' + #cols + ' from
(
select Type, Amount,
row_number() over(partition by Type order by Type, Amount) rn
from yourtable
) x
pivot
(
max(Amount)
for Type in (' + #cols + ')
) p '
execute(#query)
--drop temporary table
drop table yourtable;
I can't run the last part where I want to transpose my temporary result. I get an error for "DECLARE"
/* SQL Error (1064): You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near 'DECLARE #cols AS NVARCHAR(MAX),
#query AS NVARCHAR(MAX)
select #cols = ' at line 2 / / 2 rows affected, 2 rows found.
Duration for 3 of 4 queries: 0,000 sec. */
Can anybody help?

Answer 1

MySQL Does not have a pivot function so you will have to transpose the data from rows into columns using an aggregate function with a CASE expression:
select
sum(case when tt.type = 'UnitTest' then 1 else 0 end) UnitTest,
sum(case when tt.type = 'WebTest' then 1 else 0 end) WebTest
from test t
inner join test_type tt
on t.test_type = tt.id
See SQL Fiddle with Demo.
If you are going to have an unknown number of types that you want to convert to columns, you can use a prepared statement to generate dynamic SQL:
SET #sql = NULL;
SELECT
GROUP_CONCAT(DISTINCT
CONCAT(
'sum(CASE WHEN tt.type = ''',
type,
''' THEN 1 else 0 END) AS `',
type, '`'
)
) INTO #sql
FROM test_type;
SET #sql
= CONCAT('SELECT ', #sql, '
from test t
inner join test_type tt
on t.test_type = tt.id');
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
See SQL Fiddle with Demo