SQL查询计数记录

Question

I am making up a SQL query which will get all the transaction types from one table, and from the other table it will count the frequency of that transaction type.

My query is this:

with CTE as
(
select a.trxType,a.created,b.transaction_key,b.description,a.mode
FROM transaction_data AS a with (nolock)
     RIGHT JOIN transaction_types b with (nolock) ON b.transaction_key = a.trxType 
 )
SELECT COUNT (trxType) AS Frequency, description as trxType,mode
from CTE where created >='2017-04-11' and created <= '2018-04-13'
group by trxType ,description,mode

The transaction_types table contains all the types of transactions only and transaction_data contains the transactions which have occurred.

The problem I am facing is that even though it's the RIGHT join , it does not select all the records from the transaction_types table.

I need to select all the transactions from the transaction_types table and show the number of counts for each transaction, even if it's 0.

Please help.

Answer 1

LEFT JOIN is so much easier to follow.

I think you want:

select tt.transaction_key, tt.description, t.mode, count(t.trxType)
from transaction_types tt left join
     transaction_data t
     on tt.transaction_key = t.trxType and
        t.created >= '2017-04-11' and t.created <= '2018-04-13'
group by tt.transaction_key, tt.description, t.mode;

Notes:

• Use reasonable table aliases! a and b mean nothing. t and tt are abbreviations of the table name, so they are easier to follow.
• t.mode will be NULL for non-matching rows.
• The condition on dates needs to be in the ON clause. Otherwise, the outer join is turned into an inner join.
• LEFT JOIN is easier to follow (at least for people whose native language reads left-to-right) because it means "keep all the rows in the table you have already read".