diff --git a/app/database/jrp/repository/repository.go b/app/database/jrp/repository/repository.go
index 7cd928b6..3af62a10 100644
--- a/app/database/jrp/repository/repository.go
+++ b/app/database/jrp/repository/repository.go
@@ -174,8 +174,6 @@ func (j JrpRepository) GetAllHistory(jrpDBFilePath string) ([]*model.Jrp, error)
 		); err != nil {
 			return nil, err
 		}
-		history.Prefix = j.SqlProxy.IfNullToNullString(history.Prefix)
-		history.Suffix = j.SqlProxy.IfNullToNullString(history.Suffix)
 
 		allHistory = append(allHistory, history)
 	}
@@ -238,8 +236,6 @@ func (j JrpRepository) GetHistoryWithNumber(jrpDBFilePath string, number int) ([
 		); err != nil {
 			return nil, err
 		}
-		history.Prefix = j.SqlProxy.IfNullToNullString(history.Prefix)
-		history.Suffix = j.SqlProxy.IfNullToNullString(history.Suffix)
 
 		allHistory = append(allHistory, history)
 	}
@@ -320,8 +316,6 @@ func (j JrpRepository) SearchAllHistory(jrpDBFilePath string, keywords []string,
 		); err != nil {
 			return nil, err
 		}
-		history.Prefix = j.SqlProxy.IfNullToNullString(history.Prefix)
-		history.Suffix = j.SqlProxy.IfNullToNullString(history.Suffix)
 
 		searchedAllHistory = append(searchedAllHistory, history)
 	}
@@ -403,8 +397,6 @@ func (j JrpRepository) SearchHistoryWithNumber(
 		); err != nil {
 			return nil, err
 		}
-		history.Prefix = j.SqlProxy.IfNullToNullString(history.Prefix)
-		history.Suffix = j.SqlProxy.IfNullToNullString(history.Suffix)
 
 		searchedHistory = append(searchedHistory, history)
 	}
@@ -597,8 +589,6 @@ func (j JrpRepository) GetAllFavorite(jrpDBFilePath string) ([]*model.Jrp, error
 		); err != nil {
 			return nil, err
 		}
-		favorite.Prefix = j.SqlProxy.IfNullToNullString(favorite.Prefix)
-		favorite.Suffix = j.SqlProxy.IfNullToNullString(favorite.Suffix)
 
 		allFavorite = append(allFavorite, favorite)
 	}
@@ -661,8 +651,6 @@ func (j JrpRepository) GetFavoriteWithNumber(jrpDBFilePath string, number int) (
 		); err != nil {
 			return nil, err
 		}
-		favorite.Prefix = j.SqlProxy.IfNullToNullString(favorite.Prefix)
-		favorite.Suffix = j.SqlProxy.IfNullToNullString(favorite.Suffix)
 
 		allFavorite = append(allFavorite, favorite)
 	}
@@ -742,8 +730,6 @@ func (j JrpRepository) SearchAllFavorite(jrpDBFilePath string, keywords []string
 		); err != nil {
 			return nil, err
 		}
-		favorite.Prefix = j.SqlProxy.IfNullToNullString(favorite.Prefix)
-		favorite.Suffix = j.SqlProxy.IfNullToNullString(favorite.Suffix)
 
 		searchedAllFavorite = append(searchedAllFavorite, favorite)
 	}
@@ -825,8 +811,6 @@ func (j JrpRepository) SearchFavoriteWithNumber(
 		); err != nil {
 			return nil, err
 		}
-		favorite.Prefix = j.SqlProxy.IfNullToNullString(favorite.Prefix)
-		favorite.Suffix = j.SqlProxy.IfNullToNullString(favorite.Suffix)
 
 		searchedFavorite = append(searchedFavorite, favorite)
 	}
diff --git a/app/proxy/sql/nullstringinstance.go b/app/proxy/sql/nullstringinstance.go
index c1f5aa6f..3688bd86 100644
--- a/app/proxy/sql/nullstringinstance.go
+++ b/app/proxy/sql/nullstringinstance.go
@@ -31,9 +31,5 @@ func (n *NullStringInstance) Scan(value interface{}) error {
 
 // Value implements the driver Valuer interface.
 func (n *NullStringInstance) Value() (driver.Value, error) {
-	var v driver.Value
-	if n.FieldNullString == nil || !n.FieldNullString.Valid {
-		v = nil
-	}
-	return v, nil
+	return n.FieldNullString.String, nil
 }
diff --git a/app/proxy/sql/sqlproxy.go b/app/proxy/sql/sqlproxy.go
index 53c803e9..0feb44bc 100644
--- a/app/proxy/sql/sqlproxy.go
+++ b/app/proxy/sql/sqlproxy.go
@@ -8,7 +8,6 @@ import (
 
 // Sql is an interface for sql.
 type Sql interface {
-	IfNullToNullString(nullStringInstance *NullStringInstance) *NullStringInstance
 	Open(driverName string, dataSourceName string) (DBInstanceInterface, error)
 	StringToNullString(s string) *NullStringInstance
 }
@@ -21,19 +20,6 @@ func New() Sql {
 	return &SqlProxy{}
 }
 
-// IfNullToNullString returns a NullStringInstance if the argument is nil.
-func (*SqlProxy) IfNullToNullString(nullStringInstance *NullStringInstance) *NullStringInstance {
-	if nullStringInstance == nil {
-		nullStringInstance = &NullStringInstance{
-			FieldNullString: &sql.NullString{
-				String: "",
-				Valid:  false,
-			},
-		}
-	}
-	return nullStringInstance
-}
-
 // Open is a proxy for sql.Open.
 func (*SqlProxy) Open(driverName string, dataSourceName string) (DBInstanceInterface, error) {
 	db, err := sql.Open(driverName, dataSourceName)
diff --git a/docs/coverage.html b/docs/coverage.html
index a21c9776..9ffe0df0 100644
--- a/docs/coverage.html
+++ b/docs/coverage.html
@@ -401,10 +401,8 @@
                 ); err != nil </span><span class="cov8" title="1">{
                         return nil, err
                 }</span>
-                <span class="cov8" title="1">history.Prefix = j.SqlProxy.IfNullToNullString(history.Prefix)
-                history.Suffix = j.SqlProxy.IfNullToNullString(history.Suffix)
 
-                allHistory = append(allHistory, history)</span>
+                <span class="cov8" title="1">allHistory = append(allHistory, history)</span>
         }
 
         <span class="cov8" title="1">return allHistory, deferErr</span>
@@ -465,10 +463,8 @@
                 ); err != nil </span><span class="cov8" title="1">{
                         return nil, err
                 }</span>
-                <span class="cov8" title="1">history.Prefix = j.SqlProxy.IfNullToNullString(history.Prefix)
-                history.Suffix = j.SqlProxy.IfNullToNullString(history.Suffix)
 
-                allHistory = append(allHistory, history)</span>
+                <span class="cov8" title="1">allHistory = append(allHistory, history)</span>
         }
 
         // sort by ID asc
@@ -547,10 +543,8 @@
                 ); err != nil </span><span class="cov8" title="1">{
                         return nil, err
                 }</span>
-                <span class="cov8" title="1">history.Prefix = j.SqlProxy.IfNullToNullString(history.Prefix)
-                history.Suffix = j.SqlProxy.IfNullToNullString(history.Suffix)
 
-                searchedAllHistory = append(searchedAllHistory, history)</span>
+                <span class="cov8" title="1">searchedAllHistory = append(searchedAllHistory, history)</span>
         }
 
         <span class="cov8" title="1">return searchedAllHistory, deferErr</span>
@@ -630,10 +624,8 @@
                 ); err != nil </span><span class="cov8" title="1">{
                         return nil, err
                 }</span>
-                <span class="cov8" title="1">history.Prefix = j.SqlProxy.IfNullToNullString(history.Prefix)
-                history.Suffix = j.SqlProxy.IfNullToNullString(history.Suffix)
 
-                searchedHistory = append(searchedHistory, history)</span>
+                <span class="cov8" title="1">searchedHistory = append(searchedHistory, history)</span>
         }
 
         // sort by ID asc
@@ -824,10 +816,8 @@
                 ); err != nil </span><span class="cov8" title="1">{
                         return nil, err
                 }</span>
-                <span class="cov8" title="1">favorite.Prefix = j.SqlProxy.IfNullToNullString(favorite.Prefix)
-                favorite.Suffix = j.SqlProxy.IfNullToNullString(favorite.Suffix)
 
-                allFavorite = append(allFavorite, favorite)</span>
+                <span class="cov8" title="1">allFavorite = append(allFavorite, favorite)</span>
         }
 
         <span class="cov8" title="1">return allFavorite, deferErr</span>
@@ -888,10 +878,8 @@
                 ); err != nil </span><span class="cov8" title="1">{
                         return nil, err
                 }</span>
-                <span class="cov8" title="1">favorite.Prefix = j.SqlProxy.IfNullToNullString(favorite.Prefix)
-                favorite.Suffix = j.SqlProxy.IfNullToNullString(favorite.Suffix)
 
-                allFavorite = append(allFavorite, favorite)</span>
+                <span class="cov8" title="1">allFavorite = append(allFavorite, favorite)</span>
         }
 
         // sort by ID asc
@@ -969,10 +957,8 @@
                 ); err != nil </span><span class="cov8" title="1">{
                         return nil, err
                 }</span>
-                <span class="cov8" title="1">favorite.Prefix = j.SqlProxy.IfNullToNullString(favorite.Prefix)
-                favorite.Suffix = j.SqlProxy.IfNullToNullString(favorite.Suffix)
 
-                searchedAllFavorite = append(searchedAllFavorite, favorite)</span>
+                <span class="cov8" title="1">searchedAllFavorite = append(searchedAllFavorite, favorite)</span>
         }
 
         <span class="cov8" title="1">return searchedAllFavorite, deferErr</span>
@@ -1052,10 +1038,8 @@
                 ); err != nil </span><span class="cov8" title="1">{
                         return nil, err
                 }</span>
-                <span class="cov8" title="1">favorite.Prefix = j.SqlProxy.IfNullToNullString(favorite.Prefix)
-                favorite.Suffix = j.SqlProxy.IfNullToNullString(favorite.Suffix)
 
-                searchedFavorite = append(searchedFavorite, favorite)</span>
+                <span class="cov8" title="1">searchedFavorite = append(searchedFavorite, favorite)</span>
         }
 
         // sort by ID asc
@@ -2799,12 +2783,8 @@
 
 // Value implements the driver Valuer interface.
 func (n *NullStringInstance) Value() (driver.Value, error) <span class="cov8" title="1">{
-        var v driver.Value
-        if n.FieldNullString == nil || !n.FieldNullString.Valid </span><span class="cov8" title="1">{
-                v = nil
-        }</span>
-        <span class="cov8" title="1">return v, nil</span>
-}
+        return n.FieldNullString.String, nil
+}</span>
 </pre>
 		
 		<pre class="file" id="file29" style="display: none">package sqlproxy
@@ -2901,7 +2881,6 @@
 
 // Sql is an interface for sql.
 type Sql interface {
-        IfNullToNullString(nullStringInstance *NullStringInstance) *NullStringInstance
         Open(driverName string, dataSourceName string) (DBInstanceInterface, error)
         StringToNullString(s string) *NullStringInstance
 }
@@ -2914,19 +2893,6 @@
         return &amp;SqlProxy{}
 }</span>
 
-// IfNullToNullString returns a NullStringInstance if the argument is nil.
-func (*SqlProxy) IfNullToNullString(nullStringInstance *NullStringInstance) *NullStringInstance <span class="cov8" title="1">{
-        if nullStringInstance == nil </span><span class="cov8" title="1">{
-                nullStringInstance = &amp;NullStringInstance{
-                        FieldNullString: &amp;sql.NullString{
-                                String: "",
-                                Valid:  false,
-                        },
-                }
-        }</span>
-        <span class="cov8" title="1">return nullStringInstance</span>
-}
-
 // Open is a proxy for sql.Open.
 func (*SqlProxy) Open(driverName string, dataSourceName string) (DBInstanceInterface, error) <span class="cov8" title="1">{
         db, err := sql.Open(driverName, dataSourceName)