Remove Value::String

Its existence alongside SingleQuotedString simply doesn't make sense:
`'a string'` is a string literal, while `a string` is not a "value".

It's only used in postgresql-specific tab-separated-values parser to
store the string representation of a field's value. For that use-case
Option<String> looks like a more appropriate choice than Value.

src/sqlast/mod.rs

@@ -103,7 +103,7 @@ pub enum ASTNode {
         /// COLUMNS
         columns: Vec<String>,
         /// VALUES a vector of values to be copied
-        values: Vec<Value>,
+        values: Vec<Option<String>>,
     },
     /// UPDATE
     SQLUpdate {
@@ -290,7 +290,7 @@ impl ToString for ASTNode {
                         "\n{}",
                         values
                             .iter()
-                            .map(|v| v.to_string())
+                            .map(|v| v.clone().unwrap_or("\\N".to_string()))
                             .collect::<Vec<String>>()
                             .join("\t")
                     );

src/sqlast/value.rs

@@ -2,15 +2,13 @@ use chrono::{offset::FixedOffset, DateTime, NaiveDate, NaiveDateTime, NaiveTime}
 
 use uuid::Uuid;
 
-/// SQL values such as int, double, string timestamp
+/// SQL values such as int, double, string, timestamp
 #[derive(Debug, Clone, PartialEq)]
 pub enum Value {
     /// Literal signed long
     Long(i64),
     /// Literal floating point value
     Double(f64),
-    /// Unquoted string
-    String(String),
     /// Uuid value
     Uuid(Uuid),
     /// 'string value'
@@ -34,7 +32,6 @@ impl ToString for Value {
         match self {
             Value::Long(v) => v.to_string(),
             Value::Double(v) => v.to_string(),
-            Value::String(v) => v.to_string(),
             Value::Uuid(v) => v.to_string(),
             Value::SingleQuotedString(v) => format!("'{}'", v),
             Value::Boolean(v) => v.to_string(),

src/sqlparser.rs

@@ -690,7 +690,7 @@ impl Parser {
 
     /// Parse a tab separated values in
     /// COPY payload
-    fn parse_tsv(&mut self) -> Result<Vec<Value>, ParserError> {
+    fn parse_tsv(&mut self) -> Result<Vec<Option<String>>, ParserError> {
         let values = self.parse_tab_value()?;
         Ok(values)
     }
@@ -699,17 +699,17 @@ impl Parser {
         Ok(ASTNode::SQLValue(self.parse_value()?))
     }
 
-    fn parse_tab_value(&mut self) -> Result<Vec<Value>, ParserError> {
+    fn parse_tab_value(&mut self) -> Result<Vec<Option<String>>, ParserError> {
         let mut values = vec![];
         let mut content = String::from("");
         while let Some(t) = self.next_token_no_skip() {
             match t {
                 Token::Whitespace(Whitespace::Tab) => {
-                    values.push(Value::String(content.to_string()));
+                    values.push(Some(content.to_string()));
                     content.clear();
                 }
                 Token::Whitespace(Whitespace::Newline) => {
-                    values.push(Value::String(content.to_string()));
+                    values.push(Some(content.to_string()));
                     content.clear();
                 }
                 Token::Backslash => {
@@ -718,7 +718,7 @@ impl Parser {
                     }
                     if let Some(token) = self.next_token() {
                         if token == Token::Identifier("N".to_string()) {
-                            values.push(Value::Null);
+                            values.push(None);
                         }
                     } else {
                         continue;