Using Counts to Determine Whether Values Are Unique
Problem
You want to know whether table values are unique.
Solution
Use HAVING
in conjunction
with COUNT()
.
Discussion
DISTINCT
eliminates
duplicates but doesn’t show which values actually were duplicated in
the original data. You can use HAVING
to find unique values in situations
to which DISTINCT
does not apply.
HAVING
can tell you which values
were unique or nonunique.
The following statements show the days on which only one driver
was active, and the days on which more than one driver was active.
They’re based on using HAVING
and
COUNT()
to determine which
trav_date
values are unique or
nonunique:
mysql>SELECT trav_date, COUNT(trav_date)
->FROM driver_log
->GROUP BY trav_date
->HAVING COUNT(trav_date) = 1;
+------------+------------------+ | trav_date | COUNT(trav_date) | +------------+------------------+ | 2006-08-26 | 1 | | 2006-08-27 | 1 | | 2006-09-01 | 1 | +------------+------------------+ mysql>SELECT trav_date, COUNT(trav_date)
->FROM driver_log
->GROUP BY trav_date
->HAVING COUNT(trav_date) > 1;
+------------+------------------+ | trav_date | COUNT(trav_date) | +------------+------------------+ | 2006-08-29 | 3 | | 2006-08-30 | 2 | | 2006-09-02 | 2 | +------------+------------------+
This technique works for combinations of values, too. For
example, to find message sender/recipient pairs between whom only one
message was sent, look for combinations that occur only once in the
mail
table:
mysql>SELECT srcuser, dstuser ...
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