mysql：
select count(*) from main_index;
但是这个在这里却报语法错误。
第一种方法：
查文档得：
Aggregate functions (AVG(), MIN(), MAX(), SUM()) in column list clause are supported. Arguments to aggregate functions can be either plain attributes or arbitrary expressions. COUNT(*) is implicitly supported as using GROUP BY will add @count column to result set. Explicit support might be added in the future. COUNT(DISTINCT attr) is supported. Currently there can be at most one COUNT(DISTINCT) per query and an argument needs to be an attribute. Both current restrictions on COUNT(DISTINCT) might be lifted in the future.
也就是说只有在group by的时候才能用count(*)，如：
下面我们来说一下show meta:
SHOW META shows additional meta-information about the latest query such as query time and keyword statistics:
也就是说它显示的是最近一次查询附加的一些信息，比如查询时间、关键字统计、总记录等。
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