如何在MySQL上构建SQL查询以检索GROUP BY统计信息

Question

I have two tables, series and cards (one to many: each series has many cards)

series: seriesID, seriesName...
cards: cardID, seriesID, cardActive (bit), cardUsed(bit)

I want to return a query of:

• how many total cards does each series have?
• how many inactive cards does each series have?
• how many used cards does each series have?
• how many unused and active cards does each series have?

The goal is to have a table with the following headers:

seriesID | seriesName | total cards count | inactive cards | etc..

I've tried using this:

SELECT seriesID,COUNT(*) AS cardsCount FROM cards GROUP BY seriesID

but I wonder whether I would need 5 different queries to get the specified statistics. Should I use a function?

Answer 1

You can probably SUM the bit-fields (unless that produces a parity value rather than an INTEGER), and the CASE expression helps with the last:

 SELECT s.SeriesID,
        s.SeriesName,
        COUNT(*)              AS "Total Cards",
        SUM(NOT c.CardActive) AS "Inactive Cards",
        SUM(NOT c.CardUsed)   AS "Unused Cards",
        SUM(CASE WHEN c.CardActive AND NOT c.CardUsed THEN 1 ELSE 0 END)
                              AS "Active and Unused"
   FROM Series AS s
   JOIN Cards  AS c ON s.SeriesID = c.SeriesID
  GROUP BY s.SeriesID, s.SeriesName;

Given that it is MySQL, you could omit s.SeriesName from the GROUP BY and you'd probably end up with the same answer; most other DBMS would demand both non-aggregate columns in the GROUP BY clause.

There's a chance that the SUM(NOT c.CardActive) and SUMN(NOT c.CardUsed) expressions won't do what I want. In that case:

       SUM(CASE WHEN c.CardActive THEN 0 ELSE 1 END) AS "Inactive Cards",
       SUM(CASE WHEN c.CardUsed   THEN 0 ELSE 1 END) AS "Unused Cards",

will do the trick.

This is all clean for the series which have at least one card listed. If you have series where there are no cards and you want them in the output, then you need a LEFT OUTER JOIN between the tables, and you have to worry about nulls. This time, the SUM(CASE...END) is going to be needed throughout. Note that COUNT(C.CardID) only counts the non-null values, so this will give zero when the series has no cards.

 SELECT s.SeriesID,
        s.SeriesName,
        COUNT(c.CardID) AS "Total Cards",
        SUM(CASE WHEN c.CardID IS NULL THEN 0 WHEN c.CardActive THEN 0 ELSE 1 END)
                        AS "Inactive Cards",
        SUM(CASE WHEN c.CardID IS NULL THEN 0 WHEN c.CardUsed   THEN 0 ELSE 1 END)
                        AS "Unused Cards",
        SUM(CASE WHEN c.CardID IS NULL THEN 0
                 WHEN c.CardActive AND NOT c.CardUsed THEN 1 ELSE 0
            END)        AS "Active and Unused"
   FROM Series AS s
   LEFT JOIN Cards AS c ON s.SeriesID = c.SeriesID
  GROUP BY s.SeriesID, s.SeriesName;