SQL技巧

1.字段合并

sqlite3: select (id||'_'||name) as k from user;

2.分组统计后再筛选

select * from (SELECT count(*) as c,sn FROM t group by sn) as k where k.c>1  //此时必须用as指定c列,否则会找不到c列.

3. 数据类型转换

CAST ( expression AS data_type ) : 将参数expr转换为target_type类型

#下面data原先是char类型

SELECT * FROM my_data where CAST(data AS real)>15

4.sqlite replace into用于update时,显示成功但数据没变.

表的字段必须完整,否则缺失字段会用"(null)"填充.