6.7 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
that—but 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 |
+-------------+
|
