CAML Query

CAML (Collaborative Application Markup Language) is the XML-based language that will help you to perform a data manipulation task in SharePoint. It is easy to relate a list to CAML if you compare it to a database table and query.

CAML Query Comparison Operators:

• Contains - Contains a given text value 
• Eq - Equal to
• Geq - Greater than or equal to
• Gt - Greater than
• Leq - Less than or equal to
• Lt - Less than
• Neq - Not equal to
• DateRangesOverlap - Compares dates in recurring events to determine if they overlap
• IsNotNull - Is not null
• IsNull  - Is null

CAML Query Basic Example:
2 conditions

        camlQuery.ViewXml = @"<View>

                                   <Query>

                                       <Where>

                                        <And>

                                          <And>

                                                <Eq>

                                                   <FieldRef Name='AssignedTo'/> <Value Type='User'>" + AssignTo + @"</Value>

                                               </Eq>  

                                           </And>

                                                <Leq>

                                                  <FieldRef Name='Priority' /> <Value Type='Text'>" + Priority + @"</Value>

                                                </Leq>

                                      </And>                                 

                                     </Where>

                                   </Query>

                                 </View>";

 3 conditions

        camlQuery.ViewXml = @"<View>

                                   <Query>

                                       <Where>

                                        <And>

                                          <And>

                                                <Eq>

                                                   <FieldRef Name='AssignedTo'/> <Value Type='User'>" + AssignTo + @"</Value>

                                               </Eq>                                    

                                                <Leq>

                                                    <FieldRef Name='Created'/>

                                                    <Value Type='DateTime' IncludeTimeValue='TRUE' >" + Created.ToString("yyyy-MM-ddTHH:mm:ssZ") + @"</Value>    

                                                </Leq>

                                          </And>

                                               <Geq>

                                                <FieldRef Name='Created'/>

                                                <Value Type='DateTime' IncludeTimeValue='TRUE' >" + Created.ToString("yyyy-MM-ddTHH:mm:ssZ") + @"</Value>    

                                              </Geq>

                                      </And>                                 

                                     </Where>

                                   </Query>

                                 </View>";

 

  4 conditions

camlQuery.ViewXml = @"<View>

                                   <Query>

                                       <Where>

                                        <And>

                                          <And>

                                        <Eq>

                                           <FieldRef Name='Reference'/> <Value Type='Text'>" + Reference + @"</Value>    

                                        </Eq>

                                        <Eq>

                                           <FieldRef Name='AssignedTo'/> <Value Type='User'>" + AssignTo + @"</Value>

                                       </Eq>

                                         </And>

                                          <And>

                                        <Eq>

                                        <FieldRef Name='Priority' /> <Value Type='Text'>" + Priority + @"</Value>

                                         </Eq>

                                      <Eq>

                                           <FieldRef Name='Reference'/> <Value Type='Text'>" + Reference + @"</Value>

                                       </Eq>

                                        </And>

                                   </And>

                                     </Where>

                                   </Query>

                              </View>";

 Crete multi condition Query

    private static string CreateCAMLConditions(List<string> conditions, string type)

    {

        // No conditions => empty response

        if (conditions.Count == 0) return "";

        // Initialize temporary variables

        string typeStart = (type == "And" ? "<And>" : "<Or>");

        string typeEnd = (type == "And" ? "</And>" : "</Or>");

        // Build hierarchical structure

        while (conditions.Count >= 2)

        {

            List<string> complexConditions = new List<string>();

            for (int i = 0; i < conditions.Count; i += 2)

            {

                if (conditions.Count == i + 1) // Only one condition left

                    complexConditions.Add(conditions[i]);

                else // Two condotions - merge

                    complexConditions.Add(typeStart + conditions[i] + conditions[i + 1] + typeEnd);

            }

            conditions = complexConditions;

        }

        return conditions[0];

    }

References:
 1. http://sharepoint-works.blogspot.com/2012/05/caml-query-tutorial-for-sharepoint.html#.UclenpyGa3V
2. http://buli.waw.pl/caml-query-multiple-conditions-in-and-or-clause/