Selecting rows from other tables based on the first table using SQL - sql

Question

I have three T-SQL statements that I'd like to combine into one, so it is just a single call to the database, not three.
SELECT * FROM Clients
The first one, selects every client from the Clients table.
SELECT * FROM History
The second one, selects all the history entries from the History table. I then use some code to find the first history for each client. i.e. first history in the table for ClientID gets set into the HasHistory column for that ClientID.
SELECT * FROM Actions
The final one, I get all the actions from the action table. I then use some code to find the last action for each client. i.e. last action in the table for ClientID gets set into the LastAction column for that ClientID.
So I'm wondering if there is a way to write an SQL statement like this for example? Note this is not real SQL, just pseudo code to illustrate what I'm trying to achieve.
SELECT *
FROM Clients
AND
SELECT First History Row
FROM History
WHERE History.ClientID = Clients.ClientID
AND
SELECT Last Action Row
FROM Actions
WHERE Actions.ClientID = Clients.ClientID

Answer 1

There are a number of ways you can do this, but here is one example. I'll work on it a bit at a time to explain what we are doing. You haven't shown us the table design, so the column names are a guess, but you should get the idea.
First, you have to somehow mark which history rows you care about. One way to do this is to do a query that puts an order number on every history row, that starts from 1 with every new client, and orders them by date. This way, the first history row for each client (the one you want) always has a row number of one. This would look something like
SELECT
*,
ROW_NUMBER() OVER (PARTITION BY clientID ORDER BY historyDate) AS orderNo
FROM
History
You would do something similar with actions, except you want the latest action, not the first one, so your order by column has to be in reverse order - you do this by telling the ORDER BY to use descending order, something like this
SELECT
*,
ROW_NUMBER() OVER (PARTITION BY clientID ORDER BY actionDate DESC) AS orderNo
FROM Actions
You should now have two queries where the only rows you want are marked with a order number of one. What you do now is start with your first query, and join to these other two queries so that you only join to the orderno = 1 rows. Then all the data you want will be available in one row. You have to decide which join type to use - an inner join will only return Clients that actually have a history and an action. If you want to see clients that have no rows at all in the other tables, you need to use a left outer join. But your final query (you only need this one) will look something like
SELECT
C.*, H.*, A.*
FROM
Clients C
LEFT OUTER JOIN
(SELECT
*,
ROW_NUMBER() OVER (PARTITION BY clientID ORDER BY historyDate) AS orderNo
FROM History) H ON H.clientID = C.clientID AND H.orderNo = 1
LEFT OUTER JOIN
(SELECT
*,
ROW_NUMBER() OVER (PARTITION BY clientID ORDER BY actionDate DESC) AS orderNo
FROM Actions) A ON A.clientID = C.clientID AND A.orderNo = 1
What this says is: take Clients (which we'll call C), then for each row, try and join to (match a row from) the History query we looked at above (which we'll call H) where the client ID is the same and the orderNo is 1 - ie the first history row. It also does the same for the Actions query.