官方参考文档：SQLite Query Language: Core Functions http://www.sqlite.org/lang_corefunc.html
测试SQL语句：
SELECT substr(‘2012-12-12 10:11:12 fff‘,0,20) AS substr_0_20, substr(‘2012-12-12 10:11:12 fff‘,0,19) AS substr_0_19, substr(‘2012-12-12 10:11:12 fff‘,1,19) AS substr_1_19, substr(‘2012-12-12 10:11:12 fff‘,19) AS substr_19
测试语句结果：substr_0_20 和 substr_1_19结果保持一致；substr_0_19不知道为啥会少一位？
| substr_0_20 | substr_0_19 | substr_1_19 | substr_19 |
| 2012-12-12 10:11:12 | 2012-12-12 10:11:1 | 2012-12-12 10:11:12 | 2 fff |
经过同事的测试，在IOS上substr_0_20 无法取到正常的结果。
原文：http://www.cnblogs.com/AsionTang/p/4397833.html