Controlling String Case Sensitivity for MIN( ) and MAX( )

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 |
+------+