4.1 IFNULL 与 NULLIF
1.IFNULL(expr1,expr2):假如expr1不为NULL，则返回expr1;否则返回expr2。
2.NULLIF(expr1,expr2):如果expr1 = expr2，那么返回NULL，否则返回expr1。
实例:
MariaDB[study_db]> SELECT book_author_name,
-> IFNULL(book_author_name, '匿名') AS '作者'
-> FROM new_books
-> WHERE book_id > 20;
+------------------+----------------+
| book_author_name |作者|
+------------------+----------------+
|司马迁|司马迁|
| Ken Follett | Ken Follett |
|马克.李维|马克.李维|
| NULL |匿名|
+------------------+----------------+
4 rows in set (0.01 sec)
MariaDB[study_db]> SELECT NULLIF(1, 2), NULLIF(1, 1);
+--------------+--------------+
| NULLIF(1, 2) | NULLIF(1, 1) |
+--------------+--------------+
| 1 | NULL |
+--------------+--------------+
1 row in set (0.00 sec)
4.2 IF流程-- IF(expr1,expr2,expr3)
如果expr1为真,返回expr2,否则返回expr3;
实例
MariaDB [study_db]>SELECTIF(1>2,'Y','N');
+---------------------+
|IF(1>2,'Y','N') |
+---------------------+
| N |
+---------------------+
1rowinset (0.00sec)
4.3 CASE..THEN..END
先看CASE的用法：将case的值（或数据库里的字段）与条件进行匹配判断，如为真则将值置为THEN后面的值，命令表达式:
1.CASEvalue WHEN [compare-value]THENresult[WHEN [compare-value]THENresult...] [ELSEresult]END
2.CASEWHEN [condition]THENresult[WHEN [condition]THENresult...] [ELSEresult]END
实例代码
MariaDB [study_db]> SELECT
-> CASE
-> WHEN book_price< 40 THEN 'A'
->WHEN book_price > 50 AND book_price< 100 THEN 'B'
->WHEN book_price > 100 THEN 'C'
-> ELSE 'D'
-> END
-> AS '价格分类',
-> book_price FROM new_books;
+--------------+------------+
|价格分类|book_price|
+--------------+------------+
|D|44.00|
|D|44.44|
|B|79.00|
|A|26.00|
|A|26.00|
|A|26.60|
|D|40.00|
|A|30.00|
|D|44.00|
|B|80.00|
|C|123.00|
|A|30.00|
|D|NULL|
