diff --git a/unittest.sql b/unittest.sql
new file mode 100644
index 0000000..60bec6d
--- /dev/null
+++ b/unittest.sql
@@ -0,0 +1,31 @@
+-- Unit tests
+--
+-- Everything run together should output 1. When 0, a test failed. In that case
+-- run logically-independent groups of assertions to spot the bug.
+
+-- levenshtein
+select 0 = levenshtein(null, null) union
+select 0 = levenshtein(null, '') union
+select 0 = levenshtein('', null) union
+select 0 = levenshtein('', '') union
+
+
+-- levenshtein_k
+select 0 = levenshtein_k(null, null, 0) union
+select 0 = levenshtein_k(null, '', 0) union
+select 0 = levenshtein_k('', null, 0) union
+select 0 = levenshtein_k('', '', 0) union
+
+select 0 = levenshtein_k('p', 'p', 0) union
+select 1 = levenshtein_k('p', 'c', 0) union
+select 2 = levenshtein_k('aa', 'bb', 2) union
+select 2 = levenshtein_k('aa', 'bb', 999) union
+select 2 = levenshtein_k('aa', 'bb', 1) union
+select 2 = levenshtein_k('aa', 'bbbb', 1) union
+
+
+-- levenshtein_ratio
+select 0 = levenshtein_ratio(null, null) union
+select 0 = levenshtein_ratio(null, '') union
+select 0 = levenshtein_ratio('', null) union
+select 0 = levenshtein_ratio('', '')