Left join on dates all dates - sql

Question

I have several tables with dates that I'm trying to join to to make a large table where the data is grouped by date.
I'm accomplishing this right now by LEFT JOIN'ing to subselect's generated from the tables that I need to join to ( a lot of them are the same table with different where queries and involve SUM and COUNT so I think I have to use subselects ). The problem that I'm having is that if one of the dates doesn't existing in the first table then it doesn't show up in the table even if there are rows in subsequent tables that it's joined to with that date. I'm joining based upon DATE(datetime_column).
So it's like
SELECT date, col 1
FROM a
LEFT JOIN (SELECT date, col2 FROM a1) a2 ON DATE(a.date)=DATE(a2.date)
LEFT JOIN (SELECT date, col3 FROM a3) a4 ON DATE(a3.date)=DATE(a4.date)
Make sense? Probably not..

Answer 1

There are basically two ways to do so:
You can use a FULL OUTER JOIN
Full outer join
Conceptually, a full outer join combines the effect of applying both
left and right outer joins. Where records in the FULL OUTER JOINed
tables do not match, the result set will have NULL values for every
column of the table that lacks a matching row. For those records that
do match, a single row will be produced in the result set (containing
fields populated from both tables).
...
Some database systems do not support the full outer join functionality
directly, but they can emulate it through the use of an inner join and
UNION ALL selects of the "single table rows" from left and right
tables respectively. The same example can appear as follows:
SELECT employee.LastName, employee.DepartmentID, department.DepartmentName, department.DepartmentID
FROM employee
INNER JOIN department ON employee.DepartmentID = department.DepartmentID
UNION ALL
SELECT employee.LastName, employee.DepartmentID, CAST(NULL AS VARCHAR(20)), CAST(NULL AS INTEGER)
FROM employee
WHERE NOT EXISTS (SELECT * FROM department WHERE employee.DepartmentID = department.DepartmentID)
UNION ALL
SELECT CAST(NULL AS VARCHAR(20)), CAST(NULL AS INTEGER),
department.DepartmentName, department.DepartmentID
FROM department
WHERE NOT EXISTS (SELECT * FROM employee WHERE employee.DepartmentID = department.DepartmentID)
Other-ways you can make a master view, witch contains all the distinct keys of all the tables, to LEFT JOIN with all the tables.
select *
from (
SELECT date
FROM a
union
SELECT date
FROM a1
union
SELECT date
FROM a3
)
LEFT JOIN a using (date)
LEFT JOIN a1 using (date)
LEFT JOIN a3 using (date)
Sometime I prefer the second way to the FULL OUTER JOIN because FULL OUTER JOIN is not supported on many RDBMS and because there many of those who support it that do not optimize it well, Oracle's current version for example just threats a full outer join as the equivalent query showed in the citation, witch is very lossy for performances.

Answer 2

Try using the OUTER JOIN to fetch all the records from main table and only matching records from the sub/child table.
SELECT a.Col1, b.Col1 FROM a LEFT OUTER JOIN b ON a.Col2=b.Col2
Refer Join (SQL) for details on Joins.

Answer 3

You have another option for this, which is not using a join at all. You can bring the results together using unions and aggregations:
SELECT date, max(col1) as col1, max(col2) as col2, max(col3) as col3
FROM ((select date, col1, NULL as col2, NULL as col3 from a1) union all
(SELECT date, NULL, col2, NULL FROM a2) union all
(SELECT date, NULL, NULL, col3 FROM a3)
) t
group by date
Often the solution is the second one given by Alessandro (the first version is very cumbersome). One caveat. His solution pulls the dates from the data. Sometimes you want to generate the master list, perhaps from a calendar table or perhaps by generating the list of dates (the specifics for that depend entirely on the database).