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Obtaining a specific sort sequence based on a set of given values is
easily accomplished with a CASE expression. The basic strategy is to "assign"
specific sorting values to each row in the ORDER BY clause, based on the value
of some column or columns in each row.
Note that the value assigned is not actually in the row.
order
by case when projectid = 3 then 56
when projectid = 1 then 57
when projectid = 2 then 58
else null end
Let me make a few comments about this. First, the CASE expression
will evaluate to either 56, 57, 58 (or NULL). These values were chosen
simply to achieve a sequence. They could just as easily have been:
order
by case when projectid = 3 then 1
when projectid = 1 then 2
when projectid = 2 then 3
else null end
However, by assigning 1, 2, and 3 to 3, 1, and 2, many of you
might have been hopelessly confused. The point is, the values that
are assigned are not actually in the row, they are like a virtual column. Each
row is "assigned" or "tagged with" the value of the CASE expression, which is
calculated based on the value of the projectid column in that row.
It doesn't matter that the CASE expression isn't actually part of the
row, because you can still sort on it.
These values, 56, 57, 58 (or NULL), which were "assigned"
to each row, could have been anything. Another example might be:
order
by case when projectid = 3 then 'Curly'
when projectid = 1 then 'Larry
when projectid = 2 then 'Moe'
else null end
This CASE expression works exactly the same way as the previous two.
The row with ProjectID 3 will sort ahead of the row with ProjectID 1,
which will sort ahead of the row with ProjectID 2.
So the basic strategy is to write a CASE expression in the ORDER BY clause
to assign a value to each row which will result in that row being sorted into
the desired sequence.
But what's with this ELSE NULL business? Well, that's just my
way of ensuring data integrity. Since NULLs always sort first, we want any rows
where ProjectID isn't 3, 1, or 2 to be at the top of the list. If
there actually were such a row, it would obviously be a data integrity problem,
and we would need to action it!
Actually, ELSE NULL is redundant. If the ELSE clause is missing from the
CASE expression, NULL is assumed. But my coding style is to write it anyway.
That makes it easy to go in and change it to 59 or 'Shemp' if I decide that
rows where projectid isn't 3, 1, or 2 should sort last.
One final word. In MySQL, there's a lovely function called FIELD
that can be used here. If you use MySQL, do look it up in the manual. Its
elegance can be seen from this example:
select *
from tblproject
where projectid in (3,1,2)
order
by field(projectid,3,1,2)
If you're constructing the SQL with a scripting language (e.g. PHP, ColdFusion),
you can see immediately how this simplifies life greatly.
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