首先，什么情况下要用到组合查询呢？总的来说，有两种情况：一是在单个查询中从不同的表返回类似结构的数据；二是对单个表执行多个查询，按单个查询返回数据。在这里，我们说的组合查询是指第二种情况，即要查询的表是固定的，查询条件是不定的并且有多个查询条件。

      从例子来看，

例如，输入相应的查询条件

1、教师不等于“0”

2、机器号等于“yang” 

两个条件是“与”的关系，我们可以很容易的写出它的sql语句

select * from T_Worklog_Info  where id <> ‘0‘ and computer=‘yang‘

所以接下来要做的核心就是拼出这条语句。

在U层，我们把输入的查询条件的信息赋给实体，然后把实体传入B层。

 Dim student As New Login.Model.CmbQueryInfo
        Dim Bquery As New Login.BLL.cmbQueryforBLL
        '定义控件基类
        Dim controlArray(2) As System.Windows.Forms.Control
        Dim table As New DataTable
        Dim i As Integer

        controlArray(0) = cmbWord1
        controlArray(1) = cmbOperator1
        controlArray(2) = txtContent1
        '字段名
        student.FileName1 = cmbWord1.Text
        student.FileName2 = cmbWord2.Text
        student.FileName3 = cmbWord3.Text
        '操作符
        student.Operator1 = cmbOperator1.Text
        student.Operator2 = cmbOperator2.Text
        student.Operator3 = cmbOperator3.Text
        '查询内容
        student.QueryContent1 = txtContent1.Text
        student.QueryContent2 = txtContent2.Text
        student.QueryContent3 = txtContent3.Text
        '组合关系
        student.CompositionRelation1 = cmbRelation1.Text
        student.CompositionRelation2 = cmbRelation2.Text

        DataGridView1.Rows.Clear()
        '查询
        table = Bquery.cmbQuery(student, "T_Worklog_Info")

在B层，接收实体之后，开始拼接字符串。

首先，会用到两个方法。

ModifyFields方法主要是把“卡号”等中文字符转换为数据库中的字段“cardno”。

    Public Function ModifyFields(student As Login.Model.CmbQueryInfo) As Login.Model.CmbQueryInfo
        Dim str As String
        Select Case student.FileName1
            Case "卡号"
                str = "cardno"
            Case "学号"
                str = "studentno"
            Case "姓名"
                str = "studentname"
            Case "性别"
                str = "sex"
            Case "系别"
                str = "apartment"
            Case "年级"
                str = "grade"
            Case "班级"
                str = "class"
            Case "上机日期"
                str = "ondate"
            Case "上机时间"
                str = "ontime"
            Case "下机日期"
                str = "offdate"
            Case "下机时间"
                str = "offtime"
            Case "机房号"
                str = "computer"
            Case "机器号"
                str = "computer"
            Case "登录日期"
                str = "LoginDate"
            Case "登录时间"
                str = "LoginTime"
            Case "注销日期"
                str = "LogoutDate"
            Case "注销时间"
                str = "LogoutTime"
            Case "教师"
                str = "id"
            Case Else
                str = ""
        End Select
        student.FileName1 = str

        'student.FileName2, student.FileName3类似

        Select Case student.CompositionRelation1
            Case "与"
                str = "and"
            Case "或"
                str = "or"
            Case Else
                str = "NO"
        End Select
        student.CompositionRelation1 = str
        Select Case student.CompositionRelation2
            Case "与"
                str = "and"
            Case "或"
                str = "or"
            Case Else
                str = "NO"
        End Select
        student.CompositionRelation2 = str
        Return student
    End Function

 Public Function CreateQueryContent(student As Login.Model.CmbQueryInfo) As Login.Model.CmbQueryInfo
        Dim tempQueryComposition As New Login.Model.CmbQueryInfo
        Try
            tempQueryComposition.QueryContent1 = student.FileName1 + " " + student.Operator1 + "'" + student.QueryContent1 + "'"
            tempQueryComposition.QueryContent2 = student.FileName2 + " " + student.Operator2 + _
                                              "'" + student.QueryContent2 + "'"
            tempQueryComposition.QueryContent3 = student.FileName3 + " " + student.Operator3 + _
                                            "'" + student.QueryContent3 + "'"

            tempQueryComposition.CompositionRelation1 = student.CompositionRelation1
            tempQueryComposition.CompositionRelation2 = student.CompositionRelation2
            Return tempQueryComposition
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End Function

    Public Function CreateSqlString(student As Login.Model.CmbQueryInfo, tablename As String) As String
        Dim sql As String

        Try
            '没有组合关系时
            If student.CompositionRelation1.Trim = "NO" Then

                sql = "select * from " + tablename + " Where" + " " + student.FileName1 + student.Operator1 + "'" + student.QueryContent1 + "'"
            Else
                '有一个组合关系时
                sql = "select * from " + tablename + " Where" + " " + student.FileName1 + student.Operator1 + "'" + student.QueryContent1 + "'" + " " + student.CompositionRelation1 + " " + student.FileName2 + student.Operator2 + "'" + student.QueryContent2 + "'"
            End If
            '有两个组合关系时
            If student.CompositionRelation2.Trim <> "NO" Then
                sql = sql + student.CompositionRelation2 + " " + student.FileName3 + student.Operator3 + "'" + student.QueryContent3 + "'"
            End If

            Return sql
        Catch ex As Exception

        End Try
    End Function

      拼接完成后，可以放到数据库中检验一下，接下来的问题就容易多了。

最后的显示结果：

    组合查询，其本质终究还是查询。用拼接字符串的方法理解起来比较容易，但特别容易出错。一不小心sql语句就

会出错，不过调试起来也挺容易发现错误的。期待大家与我交流其他的查询方法。