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
CreateQueryContent方法
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
在D层,完成最后的sql语句的拼接。
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。
组合查询——拼接字符串
时间: 2024-11-05 13:33:08