sql query that shows me all the clients and those that did not buy on a certain date replace with null

You're posing it the other way around. Make a UPDATE and put null to all those who did not buy on that date:

UPDATE tabla
SET column1 = null
WHERE fechaorden NOT '2016-02-12';

Now you can make your selection to see who bought on that date and the rest will be null .

It is very likely that you need to do the join with clients, probably you have to add a left or right join with clients or change the innerjoin for a leftjoin and force all the clients out, to say that the date can be the date or null you can use coalesce()

link

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In such a way that coalesce(fechaorden, null) will return or the date of the order or null .

Here is a similar example

SELECT md.new_name,md.new_motivodecisionid,
  coalesce(rm.new_relacionmotivosid,'-1') as new_relacionmotivosid,
  rm.new_motivoeleccion, rm.new_motivorenuncia
FROM new_motivodecision md 
  left join new_relacionmotivos rm on rm.new_motivodecisionid = md.new_motivodecisionid
    and rm.new_opportunityid = 'xxx-xxx-xxx' Where md.statecode = 0

Here I am taking all the decision reasons whether or not they have a relationship with the Oportunity entity, with coalesce() if the entity new_relacionmotivos does not contain a new_motivoeleccionid next to Oportunityid marks the new_relacionmotivosid as '-1' , in your case it would be null

Since I do not know what structure of data you have a possible example of the case that you mention to me, it would be the following

SELECT clienteid ,productoid , coalesce(fechaorden, null)  
FROM Ventas.Ordenes INNER JOIN Ventas.DetallesOrden
ON Ordenes.ordenid = DetallesOrden.ordenid 
left join Ventas.Clientes on Ordenes.clienteid = Clientes.clienteid
Where fechaorden  ='2016-02-12';

or

SELECT clienteid ,productoid , coalesce(fechaorden, null)  
FROM Ventas.Ordenes INNER JOIN Ventas.DetallesOrden
ON Ordenes.ordenid = DetallesOrden.ordenid
Where fechaorden  ='2016-02-12';