এটি হ'ল NOT NULL:
CREATE TEMPORARY TABLE `myTab` (`notnul` FLOAT, `nul` FLOAT);
INSERT INTO `myTab` VALUES (1, NULL), (1, 2), (1, NULL), (1, 2), (1, NULL), (1, 2), (1, NULL), (1, 2), (1, NULL), (1, 2), (1, NULL), (1, 2);
SELECT * FROM `myTab`;
দেয়:
+--------+------+
| notnul | nul |
+--------+------+
| 1 | NULL |
| 1 | 2 |
| 1 | NULL |
| 1 | 2 |
| 1 | NULL |
| 1 | 2 |
| 1 | NULL |
| 1 | 2 |
| 1 | NULL |
| 1 | 2 |
| 1 | NULL |
| 1 | 2 |
+--------+------+
সূচক তৈরি করুন:
CREATE INDEX `notnul_nul` ON `myTab` (`notnul`, `nul`);
CREATE INDEX `nul_notnul` ON `myTab` (`nul`, `notnul`);
SHOW INDEX FROM `myTab`;
দেয়:
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| myTab | 1 | notnul_nul | 1 | notnul | A | 12 | NULL | NULL | YES | BTREE | | |
| myTab | 1 | notnul_nul | 2 | nul | A | 12 | NULL | NULL | YES | BTREE | | |
| myTab | 1 | nul_notnul | 1 | nul | A | 12 | NULL | NULL | YES | BTREE | | |
| myTab | 1 | nul_notnul | 2 | notnul | A | 12 | NULL | NULL | YES | BTREE | | |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
এখন নির্বাচিতদের ব্যাখ্যা করুন। মনে হচ্ছে মাইএসকিউএল সূচকটি ব্যবহার করে, এমনকি আপনি ব্যবহার করলেও NOT NULL:
EXPLAIN SELECT * FROM `myTab` WHERE `notnul` IS NOT NULL;
+----+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
| 1 | SIMPLE | myTab | index | notnul_nul | notnul_nul | 10 | NULL | 12 | Using where; Using index |
+----+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
EXPLAIN SELECT * FROM `myTab` WHERE `nul` IS NOT NULL;
+----+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
| 1 | SIMPLE | myTab | range | nul_notnul | nul_notnul | 5 | NULL | 6 | Using where; Using index |
+----+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
তবে, তুলনা করার সময় NOT NULLএবং NULL, মনে হয় যে মাইএসকিউএল অন্যান্য সূচকগুলি ব্যবহার করার সময় পছন্দ করে NOT NULL। যদিও এটি স্পষ্টত কোনও তথ্য যোগ করে না। এটি কারণ মাইএসকিউএল NOT NULLআপনি টাইপ-কলামে দেখতে পারেন এমন একটি পরিসীমা হিসাবে ব্যাখ্যা করে । আমি নিশ্চিত না যদি কোনও কর্মক্ষেত্র থাকে:
EXPLAIN SELECT * FROM `myTab` WHERE `nul` IS NULL && notnul=2;
+----+-------------+-------+------+-----------------------+------------+---------+-------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-----------------------+------------+---------+-------------+------+--------------------------+
| 1 | SIMPLE | myTab | ref | notnul_nul,nul_notnul | notnul_nul | 10 | const,const | 1 | Using where; Using index |
+----+-------------+-------+------+-----------------------+------------+---------+-------------+------+--------------------------+
EXPLAIN SELECT * FROM `myTab` WHERE `nul` IS NOT NULL && notnul=2;
+----+-------------+-------+-------+-----------------------+------------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+-----------------------+------------+---------+------+------+--------------------------+
| 1 | SIMPLE | myTab | range | notnul_nul,nul_notnul | notnul_nul | 10 | NULL | 1 | Using where; Using index |
+----+-------------+-------+-------+-----------------------+------------+---------+------+------+--------------------------+
আমি মনে করি মাইএসকিউএলে আরও ভাল বাস্তবায়ন হতে পারে, কারণ NULLএটি একটি বিশেষ মান। সম্ভবত বেশিরভাগ লোকই NOT NULLমূল্যবোধে আগ্রহী ।
NOT NULLকলামে আগ্রহী ।