DBF数据库条件连接正确语法DBF - 编辑

Question

我有4张桌子（ROUTBOM，HARD，MAT，ROUT）。在主表ROUTBOM中，我有ID，PROD_NO和TYPE列。我必须根据TYPE和（ID或PROD_NO）进行选择。以下条件。这可以通过内部联接 - 在加入之前有条件吗？

数据库是FoxPro

using (OleDbConnection con = new OleDbConnection(@"provider=VFPOLEDB;Data source=c:\test\Data"))

数据：ROUTBOM

id      qty      prod_no    Type    Name
20322   0.15000             2       ???    get name from HARD
        2.00000 0066773     4       ???    get name from ROUT
37500   4.00000             2       ???    get name from HARD
29750   4.00000             1       ???    get name from MAT

硬表

ID      NAME
20322   H68NK0005
37500   HAS2-30XX-H HYBRID POWDER

MAT TABLE

ID      NAME
29750   NEOPRENE (McMASTER 8694K61)
02125   SPRING STEEL STRIP .008'' X .50'' X 60''

路线表

ID      NAME
0066773 L.V. DOOR ARC PROOF J4
000198  DEVEL. L.V. DOOR J4

要求的输出

id      qty      prod_no    Type    Name
20322   0.15000             2       H68NK0005
        2.00000 0066773     4       L.V. DOOR ARC PROOF J4
37500   4.00000             2       HAS2-30XX-H HYBRID POWDER
29750   4.00000             1       NEOPRENE (McMASTER 8694K61)

查询：

条件：

if id is not null and type 2 JOIN ROUTBOM.id with MAT.id to get the name of material
if id is not null and type 1 JOIN ROUTBOM.id with HARD.id to get the name of hadware
if id is empty and type 4 JOIN ROUTBOM.prod_no with ROUT.id to get the name of part

可以在一个查询中完成吗？我不知道哪里必须适用条件。一旦我进行了第一次JOIN，查询就会从HARD中获取整个数据。

查询行数据

@"SELECT    
                                    t0.Id, 
                                    t1.Name,
                                    t0.Per_Router,
                                    t0.Prod_No,
                                    t0.Rout_No,
                                    t0.Seq,
                                    t0.Type
                                    FROM {0} t0 , "Routbom", "Hardware", "Material", "Rout");

Answer 1

这是我从你有限的解释中理解的：

string sql = @"SELECT Id,Name,Per_Router,Prod_No,Rout_No,Seq,Type
from
(
    Select t0.*, t1.Name as Name 
    from RoutBom t0
    inner join Mat t1 on t0.Id = t1.Id
    where t0.type = 2
    union 
    Select t0.*, t1.Name as Name 
    from RoutBom t0
    inner join Hard t1 on t0.Id = t1.Id
    where t0.type = 1
    union
    Select t0.*, t1.Name as Name 
    from RoutBom t0
    inner join Rout t1 on t0.Prod_no = t1.Id
    where t0.type = 4 and Empty(Nvl(t0.id,''))
) tmp
";
DataTable tbl = new DataTable();
using (OleDbConnection con = new OleDbConnection(@"provider=VFPOLEDB;Data source=c:\test\Data"))
using (OleDbCommand cmd = new OleDbCommand(sql, con))
{
 con.Open();
 tbl.Load(cmd.ExecuteReader());
}
// Do something with tbl

您可能更容易使用Linq。检查Tom Brother的Linq To VFP和VFP实体框架。

这是另一种方法，也可以解决联合问题：

void Main()
{
    string sql = @"SELECT *, 
    Cast(icase(
    type = '1', (select Name from Mat t1 where t0.Id = t1.Id),
    type = '2', (select Name from Hard t1 where t0.Id = t1.Id),
    empty(nvl(id,'')) and type='4', (select Name from Rout t1 where t0.Prod_No = t1.Id),
    '') as c(100)) as Name
    from RoutBom t0";
    DataTable tbl = new DataTable();
    using (OleDbConnection con = new OleDbConnection(@"provider=VFPOLEDB;Data source=c:\Test\Data"))
    using (OleDbCommand cmd = new OleDbCommand(sql, con))
    {
        con.Open();
        tbl.Load(cmd.ExecuteReader());
    }
    // Do something with tbl
    tbl.Dump(); // you said LinqPad above
} 

Answer 2

首先，我建议您学习JOIN的“标准”SQL基础知识（不是VFP特定的）。你可以在：Inner and Outer JOINs in SQL找到一个非常好的解释

然后将其应用于您的表格。基本上，您将拥有1个“父”表和3个“子”表，这些表通过SQL语法连接到“父”。

VFP SQL语法与“通用”SQL语法略有不同，但在大多数情况下，“通用”语法工作正常。

也许是这样的：

SELECT ROUTBOM.*,;
      MAT.Name AS Mat_Name,;
      HARD.Name AS Hard_Name,;
      ROUT.Name AS Rout_Name;
   FROM ROUTBOM;
   LEFT JOIN Mat   ON  ROUTBOM.ID = MAT.ID;
   LEFT JOIN Hard   ON  ROUTBOM.ID = HARD.ID;
   LEFT JOIN Rout   ON  ROUTBOM.ID = Rout.ID;
   WHERE < all whatever conditions >;
   INTO CURSOR Results READWRITE

SELECT Results
BROWSE

请注意，这只是记下来而未经过测试，但在尝试将其应用于项目之前，您应该学习“BASIC”SQL语法。

也许你可能想去：www3schools - SQL SELECT syntax

祝好运