Question

我有五个条件需要检查（即用户是否想要使用此字段进行搜索）。有四个组合框和一个文本字段。用户可以根据需要使用任何字段或多个字段进行搜索。要检查用户选择了哪个字段，我构建了几个if和else if语句。但是当我只在两个条件下这样做时，我意识到五个条件的繁琐任务是否有更好的方法呢？

if  (cmbAgent.Text=="")
{
    if (cmbDegree.Text=="")
    {
        OleDbDataAdapter da = new OleDbDataAdapter("SELECT * FROM UniversityData", connection);
    }
    else
    {
       OleDbDataAdapter da = new OleDbDataAdapter("SELECT * FROM UniversityData WHERE Expertise LIKE '%" + cmbDegree.Text + "%' ", connection);
    }
}
else if(cmbDegree.Text=="")
{
    OleDbDataAdapter da = new OleDbDataAdapter("SELECT * FROM UniversityData WHERE SourceOfContact LIKE '%"+ cmbAgent.Text + "%' ", connection);
}
else
{
    OleDbDataAdapter da = new OleDbDataAdapter("SELECT * FROM UniversityData WHERE SourceOfContact LIKE '%" + cmbAgent.Text + "%' and Expertise LIKE '%" + cmbDegree .Text + "%' ", connection);
}

Answer 1

这就是为什么大多数单独构建查询/查询字符串。例如：

var sb = new StringBuilder();
sb.Append("Select * from UniversityData where 1 = 1");
if(!string.IsNullOrEmpty(cmbDegree.Text.Trim())){
    sb.Append(" and Expertise like '%" + cmbDegree.Text + "%'")
}
//...
var querystring = sb.ToString();
OleDbDataAdapter da = new OleDbDataAdapter(querystring);

Answer 2

如果用户需要/想要输入多个值，该怎么办？您可以轻松地动态构建查询。

顺便说一下，您应该使用查询参数来阻止SQL注入。

// the "where 1=1" allows to always concatenate "and xxx"
// instead of testing if there were fulfilled conditions before
var query = "SELECT * FROM UniversityData WHERE 1 = 1";

var parameters = new Dictionary<string, string>();

if (txtDegree.Text != "")
{
   query += " AND Expertise like '%' + ? + '%' ";
   parameters.Add("degree", txtDegree.Text);
}

if(txtAgent.Text != "")
{
    query += " AND SourceOfContact like '%' + ? + '%' ";
    parameters.Add("agent", txtAgent.Text);
}

OleDbDataAdapter da = new OleDbDataAdapter(query, connection);
// add the parameters
foreach (var p in parameters) {
    da.SelectCommande.Parameters.Add(p.Key, OleDbType.VarChar, p.Value);
}

请注意，OleDb不支持命名参数。如果可以，我会考虑切换到Sql命令和适配器。

顺便说一句，如果您曾经/可能想要使用Linq构建您的查询（例如通过Entity Framework或任何其他ORM），您也可以这样做，因为Linq和Entity Framework一起是后期的（意味着在实际读取结果之前不会执行查询）。

// build the query
var results = from ud in context.UniversityData
              select ud;

if (txtDegree.Text != string.Empty) {
    results = from ud in results
              where ud.Expertise.Contains(txtDegree.Text)
              select ud;
}

if (txtAgent.Text != string.Empty) {
    results = from ud in results
              where ud.SourceOfContact.Contains(txtAgent.Text)
              select ud;
}

// use the results
myControl.DataSource = results.ToList(); // the ToList() call actually calls the query

Answer 3

我会做这样的事情：

var query = "Select * from UniveristyData";
var wheres = new List<string>();
if (!cmbDegree.Text.IsNullOrEmpty())
    wheres.Add("Expertise like '%" + cmbDegree.Text + "%'");
if (!cmbAgent.Text.IsNullOrEmpty())
    wheres.Add("SourceOfContact like '%"+cmbAgent.Text+"%'");

if (wheres.Any())
    query += " where " + string.Join(" and ", wheres);

var da = new OleDbDataAdapter(query, connection);

Answer 4

您可以先构建一个键值列表：

List<Tuple<string, string>> keyValueList = new List<Tuple<string, string>>();

keyValueList.Add(new Tuple<string, string>("Expertise", cmbDegree.Text));
keyValueList.Add(new Tuple<string, string>("SourceOfContact", cmbAgent.Text));

依此类推，然后从keyValueList：

构造你的Where-Clause

var conditionsArray = keyValueList.Where(p => !string.IsNullOrWhiteSpace(p.Item2)).Select(q => q.Item1 + " LIKE '%" + q.Item2 + "%'").ToArray();

最后：

var sqlQuery = "SELECT * FROM UniversityData WHERE " + string.Join(" AND ", conditionsArray);

如果在添加WHERE子句之前数组为空，则需要添加检查，但我怀疑你需要帮助： - ）

要添加更多条件，您只需要向keyValueList（1行）添加更多元组，而不需要在以后更改其他代码。

几个嵌套if条件

4 个答案: