avoid insert duplicate information to database and give a hint to the user

Why not add a validation to the user name field which checks whether the entered value already exists?

would you give an example, i don't have sample code for "validation" part to study, i would be super appreciate

Something like this

if(
  a!queryEntity(
    /* Search for entered username */
  ).totalCount > 0,
  "Error",
  null
)

seems i got what you mean, but where to fill of what you said would not commit any grammer error? Such as mix key-word and non-key word, this is what i always headache with, such as write if-else part inside a!writeToDataStoreEntity(if(....)) is not appropriate because of not conform rule

Please post your code using insert -> code

what do you mean

Without seeing your code I can not help.

a!localVariables(
  local!rule:rule!usernameRule(),
a!formLayout(
  label: "Username",
  contents: {
    a!textField(
      label: "username",
      labelPosition: if(
        ri!readOnly,
        "ADJACENT",
        "ABOVE"
      ),
      value: ri!username.username,
      saveInto: ri!username.username,
      refreshAfter: "UNFOCUS",
      validations: {}
    ),
    a!textField(
      label: "gender",
      labelPosition: if(
        ri!readOnly,
        "ADJACENT",
        "ABOVE"
      ),
      value: ri!username.gender,
      saveInto: ri!username.gender,
      refreshAfter: "UNFOCUS",
      validations: {}
    ),
    a!columnsLayout(
      columns: {
        a!columnLayout(
          contents: {
            a!textField(
              label: "Password",
              labelPosition: if(
                ri!readOnly,
                "ADJACENT",
                "ABOVE"
              ),
              value: ri!username.password,
              saveInto: ri!username.password,
              characterLimit: 255,
              readOnly: ri!readOnly
            )
          }
        )
      }
    ),
  },
 
  buttons: a!buttonLayout(
    primaryButtons: {
      a!buttonWidget(
        label: "Insert",
        
        saveInto:(
          a!writeToDataStoreEntity(
            dataStoreEntity: cons!usernameConstant,
            valueToStore:(ri!username)
           ),       
           ),
           
        submit: if(ri!username.username in local!rule.username
        ,false,true),
        style: "PRIMARY",
      ),
      a!buttonWidget(
        label: "update",
        saveInto:(
          a!writeToDataStoreEntity(
            dataStoreEntity: cons!usernameConstant,
            valueToStore:ri!username,           
          ),
        ),
        submit: true,
        style: "PRIMARY"
      ),
      a!buttonWidget(
        label: "delete",      
        saveInto:(
          a!deleteFromDataStoreEntities(
            dataToDelete: {
              a!entityDataIdentifiers(
                entity: cons!usernameConstant,
                identifiers:{ri!username.username},
              )
            }
          )
        ),
        submit: true,
        style: "PRIMARY"
      )
    },
    secondaryButtons: {
      a!buttonWidget(
        label: "Cancel",
        value: true,
        saveInto: ri!cancel,
        submit: true,
        style: "NORMAL",
        validate: false
      )
    },
    showWhen: or(
      isnull(
        ri!readOnly
      ),
      not(
        ri!readOnly
      )
    )
  )
 
)
)

I am going to create a back end system, but have to be without any duplicate primary key insert into, or for registration the user will not figure out. The question only for row of 52 to 67, i tried multiple ways not let back end update any function for duplicate primary key been inserted, and let user know he have to choose a different one. But know the error report as "Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error at function 'if' [line 64]: Invalid number of parameters, function 'if' requires an odd number of parameters, passed 4.". I still have no idea what is the mistake. 

I am going to create a back end system, but have to be without any duplicate primary key insert into, or for registration the user will not figure out. The question only for row of 52 to 67, i tried multiple ways not let back end update any function for duplicate primary key been inserted, and let user know he have to choose a different one. But know the error report as "Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error at function 'if' [line 64]: Invalid number of parameters, function 'if' requires an odd number of parameters, passed 4.". I still have no idea what is the mistake. 

This is very confusing ...

ri!username.username in local!rule.username,

is no valid Appian syntax

I just want it figure out whether username fill in text blank section could be found under the database, that local!rule is searching all back end username create by a constant. What could be a standard way to do so?

Did you try adding the validation?

why my totalCount always -1, though the value do exist when use a!query?

This is when you set fetchTotalCount to false. As this is also the default, you want to set it explicitly to true.

a!localVariables(
a!formLayout(
  label: "Username",
  contents: {
    a!textField(
      label: "username",
      labelPosition: if(
        ri!readOnly,
        "ADJACENT",
        "ABOVE"
      ),
      value: ri!username.username,
      saveInto: ri!username.username,
      refreshAfter: "UNFOCUS",
      validations: {}
    ),
    a!textField(
      label: "gender",
      labelPosition: if(
        ri!readOnly,
        "ADJACENT",
        "ABOVE"
      ),
      value: ri!username.gender,
      saveInto: ri!username.gender,
      refreshAfter: "UNFOCUS",
      validations: {}
    ),
    a!columnsLayout(
      columns: {
        a!columnLayout(
          contents: {
            a!textField(
              label: "Password",
              labelPosition: if(
                ri!readOnly,
                "ADJACENT",
                "ABOVE"
              ),
              value: ri!username.password,
              saveInto: ri!username.password,
              characterLimit: 255,
              readOnly: ri!readOnly
            )
          }
        )
      }
    ),
  },
 
  buttons: a!buttonLayout(
    primaryButtons: {
      a!buttonWidget(
        label: "Insert",
        
        saveInto:(
          a!writeToDataStoreEntity(
            dataStoreEntity: cons!usernameConstant,
            valueToStore:(ri!username)
           ),       
           ),
           
        submit: if(rule!username_detail(ri!username.username)<>0,false,true),
        style: "PRIMARY",
      ),
      a!buttonWidget(
        label: "update",
        saveInto:(
          a!writeToDataStoreEntity(
            dataStoreEntity: cons!usernameConstant,
            valueToStore:ri!username,           
          ),
        ),
        submit: true,
        style: "PRIMARY"
      ),
      a!buttonWidget(
        label: "delete",      
        saveInto:(
          a!deleteFromDataStoreEntities(
            dataToDelete: {
              a!entityDataIdentifiers(
                entity: cons!usernameConstant,
                identifiers:{ri!username.username},
              )
            }
          )
        ),
        submit: true,
        style: "PRIMARY"
      )
    },
    secondaryButtons: {
      a!buttonWidget(
        label: "Cancel",
        value: true,
        saveInto: ri!cancel,
        submit: true,
        style: "NORMAL",
        validate: false
      )
    },
    showWhen: or(
      isnull(
        ri!readOnly
      ),
      not(
        ri!readOnly
      )
    )
  )
 
)
)

a!queryEntity(
  entity: cons!usernameConstant,
  query: a!query(
    pagingInfo: a!pagingInfo(
      startIndex: 1,
      batchSize: 50
    ),
    logicalexpression: a!queryLogicalExpression(
      operator: "AND",
      filters: {
        a!queryFilter(
          field: "username",
          operator: "=",
          value: ri!username
        )
      }
    ),
    aggregation: a!queryAggregation(
      aggregationColumns: {
        a!queryAggregationColumn(
          field: "username",
          isGrouping: true
        ),
        a!queryAggregationColumn(
          field: "username",
          alias: "number",
          aggregationFunction: "COUNT"
        )
      }
    ),
  ),
).data

Line 15

validations: {
  if(
    a!queryEntity(
      entity: cons!usernameConstant,
      fetchTotalCount: true,
      query: a!query(
        pagingInfo: a!pagingInfo(
          startIndex: 1,
          batchSize: 1
        ),
        filter: a!queryFilter(
          field: "username",
          operator: "=",
          value: ri!username.username
        )
      )
    ).totalCount > 0,
    "Username exists",
    null
  )
}

where should I put validation is appropriate?