true false unknown logic

docs/2022-07-31-sql.html

@@ -288,6 +288,7 @@
       <code>NULL != NULL</code>, we only can check with <code>IS NULL</code> or
       <code>IS NOT NULL</code>.
     </p>
+    <p>This is based on Kleene's TRUE-FALSE-UNKNOWN logic.</p>
     <div class="markdown-heading">
       <h2 class="heading-element"><code>VARCHAR</code> or <code>TEXT</code></h2>
       <a

posts/2022-07-31-sql.md

@@ -134,6 +134,8 @@ WHERE (field_something IS NULL OR field_something != 1)
 Because `NULL` is not equal to anything, even `NULL != NULL`, we only can check
 with `IS NULL` or `IS NOT NULL`.
 
+This is based on Kleene's TRUE-FALSE-UNKNOWN logic.
+
 ## `VARCHAR` or `TEXT`
 
 Prefer `VARCHAR` if you need to query and of course use index, and make sure