7.7 Controlling String Case Sensitivity for MIN( ) and MAX( )
7.7.1 Problem
MIN( ) and MAX( ) select
strings in case sensitive fashion when you don't
want them to, or vice versa.
7.7.2 Solution
Alter the case sensitivity of the strings.
7.7.3 Discussion
When applied
to string values, MIN( ) and MAX(
) produce results determined according to lexical sorting
rules. One factor in string sorting is case sensitivity, so
MIN( ) and MAX( ) are affected
by that as well. In Chapter 6, we used a
textblob_val table containing two columns of
apparently identical values:
mysql> SELECT tstr, bstr FROM textblob_val;
+------+------+
| tstr | bstr |
+------+------+
| aaa | aaa |
| AAA | AAA |
| bbb | bbb |
| BBB | BBB |
+------+------+
However, although the values look the same, they
don't behave the same. bstr is a
BLOB column and is case sensitive.
tstr, a TEXT column, is not. As
a result, MIN( ) and MAX( )
will not necessarily produce the same results for the two columns:
mysql> SELECT MIN(tstr), MIN(bstr) FROM textblob_val;
+-----------+-----------+
| MIN(tstr) | MIN(bstr) |
+-----------+-----------+
| aaa | AAA |
+-----------+-----------+
To make tstr case sensitive, use
BINARY:
mysql> SELECT MIN(BINARY tstr) FROM textblob_val;
+------------------+
| MIN(BINARY tstr) |
+------------------+
| AAA |
+------------------+
To make bstr not case sensitive, you can convert
the values to a given lettercase:
mysql> SELECT MIN(LOWER(bstr)) FROM textblob_val;
+------------------+
| MIN(LOWER(bstr)) |
+------------------+
| aaa |
+------------------+
Unfortunately, doing so also changes the displayed value. If
that's an issue, use this technique instead (and
note that it may yield a somewhat different result):
mysql> SELECT @min := MIN(LOWER(bstr)) FROM textblob_val;
mysql> SELECT bstr FROM textblob_val WHERE LOWER(bstr) = @min;
+------+
| bstr |
+------+
| aaa |
| AAA |
+------+
|
