每月约会，十二月失踪？

Question

SELECT month(dateofappointment), COUNT(*) 'NumberOfAppointments'
    FROM appointment 
    WHERE YEAR(dateofappointment) = '2016'  
    GROUP BY  MONTH(dateofappointment)

这显示了所有月份，但 12 月不存在，因为那一年没有任何预约。我如何将 12 月显示为 0？

Answer 1

为了解决这些类型的查询，将它们表达为一系列要求通常会有所帮助，这样可以更容易解决。

如果结果不符合预期，请在确定新要求时用新要求更新您的要求声明，然后重试：

据我所知，您现在有 2 个要求：

Return 2016 年每个月一行
每一行显示相应月份的约会计数

好吧，这很冗长，但您会发现查询中缺少的是定义“2016 年每个月的 1 行”的语句，因此您需要先构建该记录集，无论是手动还是通过递归。

MySQL does not currently support recursive Common Table Expressions, this is a trivial concept in many other RDBMSs

但是如果MySQL不支持递归，我们有什么选择呢？以下是对 SO 的一些其他尝试：

МуSQL Get a list of dates in month, year
How to generate a dynamic sequence table in MySQL?
generate an integer sequence in MySQL

这可能听起来有点 hack，但您可以在数据库中使用任何 table 超过 12 行并具有自动递增字段，哦，并且从 1 (或以下）。忘了这个是对是错，它会起作用：

SELECT Id 
FROM LogEvent -- An arbitrary table that I know has records starting from 1
WHERE Id BETWEEN 1 AND 12

所以这是 hacky，但我们可以实现一个行计数函数，这样我们就可以使用任何具有 12 行或更多行的 table，而不管 ids 或种子，从中偷走了这个：MySQL get row number on select - Answer by Mike Cialowicz

SET @rank=0;
SELECT @rank:=@rank+1 AS rank
FROM orders
WHERE rank <= 12

现在我们可以将此结果集中缺失的行联合到原始查询或使用连接运算符。使用联合的第一个解决方案。

It is common to use UNION ALL to inject missing rows to a recordset because it separates the expected result query from the exceptional or default results. Sometimes this syntax makes it easier to interpret the expected operation

SET @rank = 0;

SELECT month(dateofappointment) as Month, COUNT(*) 'NumberOfAppointments'
FROM appointment 
WHERE YEAR(dateofappointment) = '2016'  
GROUP BY  MONTH(dateofappointment) 

UNION ALL

SELECT rank, 0
FROM (
    SELECT @rank:=@rank+1 AS rank
    FROM rows
    WHERE @rank < 12
) months
WHERE NOT EXISTS (SELECT dateofappointment 
                  FROM appointment
                  WHERE YEAR(dateofappointment) = '2016' AND MONTH(dateofappointment) = months.rank)
ORDER BY Month

但这会导致一个丑陋的查询。您还可以在约会计数上使用左连接加入月份查询，但这里的意图更难识别。

SET @rank = 0;

SELECT months.rank, COUNT(appointment.dateofappointment) 
FROM (
    SELECT @rank:=@rank+1 AS rank
    FROM rows
    WHERE @rank < 12
) months
LEFT OUTER JOIN appointment ON months.rank = Month(appointment.dateofappointment) AND YEAR(dateofappointment) = '2016'
GROUP BY months.rank

I have saved these queries into a SqlFiddle so you can see the results: http://sqlfiddle.com/#!9/99d485/4

As I pointed out above, this is trivial in MS SQL and Oracle RDBMS, where we can generate sequences of values dynamically through recursive Common Table Expressions (CTEs) For the players at home here is an implementation in MS SQL Server 2014. The example is a little more evolved, using a from and to date to filter the results dynamically

-- Dynamic MS SQL Example using recursive CTE
DECLARE @FromDate Date = '2016-01-01'
DECLARE @ToDate Date = '2016-12-31'
;
WITH Months(Year, Month, Date) AS
(
    SELECT Year(@FromDate), Month(@FromDate), @FromDate
    UNION ALL
    SELECT Year(NextMonth.Date), Month(NextMonth.Date), NextMonth.Date
    FROM Months
    CROSS APPLY (SELECT DateAdd(m, 1, Date) Date) NextMonth
    WHERE NextMonth.Date < @ToDate
)
SELECT Months.Year, Months.Month, COUNT(*) as 'NumberOfAppointments'
FROM Months
LEFT OUTER JOIN appointment ON Year(dateofappointment) = Months.Year AND Month(dateofappointment) = Months.Month
GROUP BY Months.Year, Months.Month

每月约会，十二月失踪？

Appointments per month, December missing?

mysql

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