group by 和count
select count(type) from product group by type
结果统计出来的是每组的个数（多行结果）
想统计有多少种类型（单一结果）要怎么写？
除了这种写法，单一语句能完成吗？
select count(1) from (select count(type) from product group by type)

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select count(distinct type) from product
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探讨

select count(distinct type) from product