Controlling Case Sensitivity of String Sorts

6.7.1 Problem

String sorts are case sensitive when you don't want them to be, or vice versa.

6.7.2 Solution

Alter the case sensitivity of the sorted values.

6.7.3 Discussion

Chapter 4 discusses the fact that binary strings are case sensitive in comparisons, whereas non-binary strings are not. This property carries over into string sorting as well: ORDER BY produces lexical sorts that are case sensitive for binary strings and not case sensitive for non-binary strings. The following table textblob_val contains a TEXT column tstr and a BLOB column bstr that serve to demonstrate this:

mysql> SELECT * FROM textblob_val;
+------+------+
| tstr | bstr |
+------+------+
| aaa | aaa |
| AAA | AAA |
| bbb | bbb |
| BBB | BBB |
+------+------+

Both columns contain the same values. But they produce different sort results, because TEXT columns are not case sensitive and BLOB columns are:

mysql> SELECT tstr FROM textblob_val ORDER BY tstr;
+------+
| tstr |
+------+
| aaa |
| AAA |
| bbb |
| BBB |
+------+
mysql> SELECT bstr FROM textblob_val ORDER BY bstr;
+------+
| bstr |
+------+
| AAA |
| BBB |
| aaa |
| bbb |
+------+

To control case sensitivity in ORDER BY clauses, use the techniques discussed in Chapter 4 for affecting string comparisons. To perform a case-sensitive sort for strings that are not case sensitive (such as those in the tstr column) cast the sort column to binary-string form using the BINARY keyword:

mysql> SELECT tstr FROM textblob_val ORDER BY BINARY tstr;
+------+
| tstr |
+------+
| AAA |
| BBB |
| aaa |
| bbb |
+------+

Another possibility is to convert the output column to binary and sort that:

mysql> SELECT BINARY tstr FROM textblob_val ORDER BY 1;
+-------------+
| BINARY tstr |
+-------------+
| AAA |
| BBB |
| aaa |
| bbb |
+-------------+

You can also use the CAST( ) function that is available as of MySQL 4.0.2:

mysql> SELECT tstr FROM textblob_val ORDER BY CAST(tstr AS BINARY);
+------+
| tstr |
+------+
| AAA |
| BBB |
| aaa |
| bbb |
+------+

The complementary operation is to sort binary strings in non-case-sensitive fashion. To do this, convert the values to uppercase or lowercase with UPPER( ) or LOWER( ):

mysql> SELECT bstr FROM textblob_val ORDER BY UPPER(bstr);
+------+
| bstr |
+------+
| aaa |
| AAA |
| bbb |
| BBB |
+------+

Alternatively, you can convert the output column and sort thatbut doing so affects the displayed values, possibly in an undesirable way:

mysql> SELECT UPPER(bstr) FROM textblob_val ORDER BY 1;
+-------------+
| UPPER(bstr) |
+-------------+
| AAA |
| AAA |
| BBB |
| BBB |
+-------------+