Question

我目前正在尝试构建一个有点棘手的MySQL Select语句。以下是我要完成的任务：

我有一张这样的表：

data_table

uniqueID      stringID          subject
  1             144           "My Subject"
  2             144           "My Subject - New"
  3             144           "My Subject - Newest"
  4             211           "Some other column"

基本上，我想要做的是能够SELECT / GROUP BY stringID（stringID是线程化的图片）而不是重复。此外，我想选择最近的stringID行（在上面的示例中是uniqueID 3）。

因此，如果我要查询数据库，它将返回以下内容（最新的uniqueID位于顶部）：

uniqueID   stringID    subject
 4          211        "Some other column"  
 3          144        "My Subject - Newest" //Notice this is the most recent and distinct stringID row, with the proper subject column.

我希望这是有道理的。谢谢你的帮助。

Answer 1

尝试以下方法。它可能不是最有效的查询，但它可以工作：

SELECT uniqueID, stringID, subject
FROM data_table
WHERE uniqueID IN
 (
  SELECT MAX(uniqueID) 
  FROM data_table
  GROUP BY stringID
 )
ORDER BY uniqueID DESC

Answer 2

SELECT DISTINCT(a),
  ( SELECT DISTINCT(b) ) AS b,
  ( SELECT DISTINCT(c) ) AS c

FROM tblMyTBL

WHERE...
Order By...
Etc.

Answer 3

修改：根据评论中OP提供的新信息，最好依赖uniqueID：

select t.uniqueID
       , t.stringID
       , t.subject
       , t.your_timestamp_col
from   data_table t
       left outer join data_table t2
       on t.stringID = t2.stringID
    and
       t2.your_timestamp_col > t.your_timestamp_col
where  t2.uniqueID is null

如果像lexu在评论中提到的那样，您确定最高uniqueID值始终与最新主题相对应，那么您可以这样做：

select t.uniqueID
       , t.stringID
       , t.subject
from   data_table t
       left outer join data_table t2
       on t.stringID = t2.stringID
    and
       t2.uniqueID > t.uniqueID
where  t2.uniqueID is null

这基本上意味着：只返回data_table中没有更高uniqueID值的记录。

Answer 4

我有类似的情况，发现了不同的查询。试试这个：

SELECT MAX(uniqueID), stringID, subject
 FROM data_table
 GROUP BY stringID

Answer 5

private void LoadAllFamilyMembers(string relationShip)
        {
            lbFamilyMembers.SelectedIndexChanged -= new EventHandler(lbFamilyMembers_SelectedIndexChanged);
            SqlCommand cmd = new SqlCommand("select familymemberid,name from FamilyMembers where relationship = @relationship", con);
            cmd.Parameters.AddWithValue("@relationship", relationShip);
            DataTable dt = new DataTable();
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            adapter.Fill(dt);
            lbFamilyMembers.DataSource = dt;
            lbFamilyMembers.DisplayMember = "name";
            lbFamilyMembers.ValueMember = "familymemberid";
            lbFamilyMembers.SelectedIndex = -1;
            lbFamilyMembers.SelectedIndexChanged += new EventHandler(lbFamilyMembers_SelectedIndexChanged);
        }

MySQL为多列选择语句DISTINCT

5 个答案: