在 SQL 中将 INT 转换为 FLOAT

Question

I have this Query :

   SELECT sl.sms_prefix, sum( sl.parts ) , cp.country_name, CAST(SUM(sl.parts) AS NUMERIC(10,4)) * CAST(cp.price AS NUMERIC(10,4))
FROM sms_log sl, sms_transaction st, country_prefix cp
WHERE st.customer_id =1
AND st.sendtime >=1329865200
AND st.sendtime <=1330037999
AND st.sms_trans_id = sl.trans_id
AND sl.sms_prefix = cp.prefix
AND st.customer_id = cp.customer_id
GROUP BY sl.sms_prefix
LIMIT 0 , 30

Result:

sms_prefix  sum( sl.parts )     country_name    price   total
==================================================================================
45            2                        Denmark   0.01   0.019999999552965
63            3                        Philippines   2  6

As you see the "total" is not correct for Denmark because sum( sl.parts )=2 Multiply with 0.01 the total should be 0.02 .

Price field is FLOAT how I can CAST the total to float ?

Regards,

Answer 1

While @Heximal's answer works, I don't personally recommend it.

This is because it uses implicit casting. Although you didn't type CAST , either the SUM() or the 0.0 need to be cast to be the same data-types, before the + can happen. In this case the order of precedence is in your favour, and you get a float on both sides, and a float as a result of the + . But SUM(aFloatField) + 0 does not yield an INT, because the 0 is being implicitly cast to a FLOAT.

I find that in most programming cases, it is much preferable to be explicit . Don't leave things to chance, confusion, or interpretation.

If you want to be explicit, I would use the following.

CAST(SUM(sl.parts) AS FLOAT) * cp.price
-- using MySQL CAST FLOAT  requires 8.0

I won't discuss whether NUMERIC or FLOAT *(fixed point, instead of floating point)* is more appropriate, when it comes to rounding errors, etc. I'll just let you google that if you need to, but FLOAT is so massively misused that there is a lot to read about the subject already out there.

You can try the following to see what happens...

 CAST(SUM(sl.parts) AS NUMERIC(10,4)) * CAST(cp.price AS NUMERIC(10,4))

Answer 2

在 oracle db 中有一个将 int 转换为 float 的技巧（我想，它也应该在 mysql 中工作）：

select myintfield + 0.0 as myfloatfield from mytable