Menu
I need a Query that without any changes work on these three different database server : MySQL, MSSQL, PostgreSQL . In this query i have to to calculate a column with the following expression that work correctly on MySQL :
COUNT(DISTINCT field_char,field_int,field_date) AS costumernum
The fields in the distinct are of different type :
field_char = character
field_int = integer
field_date = datetime
The expression is inside a parent query select, so if i try to achieve the result with a sub query approach, i stumble in this situation :
SELECT t0.description,t0.depnum
(select count(*) from (
select distinct f1, f2, f3 from salestable t1
where t1.depnum = t0.depnum
) a) AS numitems
FROM salestable t0
I get an error with this query, how can i get the value of the parent query ?
The expression work correctly on MySQL but i get an error when i try to execute it on Sql Server or PostgreSQL (the problem is that the count function doesn't accept 3 arguments of different type on MSSQL/PostgreSQL), is there a way to achieve the same result with an expression that work in each of these database server (SQL Server, MySQL, PostgreSQL ) ?
971
MySQL DISTINCT with multiple columnsWhen you specify multiple columns in the DISTINCT clause, the DISTINCT clause will use the combination of values in these columns to determine the uniqueness of the row in the result set.
but when we want to count distinct column combinations, we must either clumsily concatenate values (and be very careful to choose the right separator): select count(distinct col1 || '-' || col2) from mytable; or use a subquery: select count(*) from (select distinct col1, col2 from mytable);
You can GROUP BY multiple columns, to get the count of each combination. Show activity on this post. Show activity on this post.
To count distinct values, you can use distinct in aggregate function count(). The result i3 tells that we have 3 distinct values in the table.
A general way to do this on any platform is as follows:
select count(*) from (
select distinct f1, f2, f3 from table
) a
Edit for new info:
What if you try joining to the distinct list (including the dept) and then doing the count? I created some test data and this seems to work. Make sure the COUNT is on one of the t1 columns - otherwise it will mistakenly return 1 instead of 0 when there are no corresponding entries in t1.
SELECT t0.description, t0.depnum, count(t1.depnum) as 'numitems'
FROM salestable t0
LEFT JOIN (select distinct f1,f2,f3,depnum from salestable) t1
ON t0.depnum = t1.depnum
GROUP BY
t0.description, t0.depnum
How about concatenating?
COUNT(DISTINCT field_char || '.' ||
cast(field_int as varchar) || '.' ||
cast(field_date as varchar)) AS costumernum
Warning: your concatenation operator may vary with RDBMS flavor.
Update
Apparently, the concatenation operator portability is question by itself:
I tried to help you with the distinct issue.
45
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
