以下是一個簡單的 SQL SELECT 解析器的 Java 實現，支持單表查詢和基本條件過濾。代碼包含詞法分析和語法分析模塊，并支持以下語法：

SELECT column1, column2 FROM table WHERE column3 = 5

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完整代碼

1. Token 類型定義 (TokenType.java)

public enum TokenType {
SELECT, FROM, WHERE,
IDENTIFIER, COMMA, STAR, EQUALS,
STRING, NUMBER, EOF
}

2. Token 類 (Token.java)

public class Token {
public final TokenType type;
public final String value;

public Token(TokenType type, String value) {this.type = type;this.value = value;
}@Override
public String toString() {return String.format("(%s, %s)", type, value);
}

}

3. 詞法分析器 (Lexer.java)

import java.util.ArrayList;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Lexer {
private final String input;
private int pos = 0;

private static final Pattern TOKEN_PATTERN = Pattern.compile("(?i)(SELECT|FROM|WHERE)" +    // 關鍵字 (不區分大小寫)"|([a-zA-Z_][a-zA-Z0-9_]*)" + // 標識符"|(\\*)" +                    // 星號"|(,)" +                      // 逗號"|(=)" +                      // 等于號"|('([^']*)')" +              // 字符串字面量"|(\\d+)" +                   // 數字"|(\\s+)"                     // 空白字符 (跳過)
);public Lexer(String input) {this.input = input;
}public ArrayList<Token> tokenize() {ArrayList<Token> tokens = new ArrayList<>();Matcher matcher = TOKEN_PATTERN.matcher(input);while (pos < input.length()) {if (!matcher.find(pos)) {throw new RuntimeException("Invalid token at position: " + pos);}// 跳過空白字符if (matcher.group(7) != null) {pos = matcher.end();continue;}// 匹配其他 Tokenfor (int i = 1; i <= matcher.groupCount(); i++) {if (matcher.group(i) != null) {TokenType type = null;String value = matcher.group(i);switch (i) {case 1:  // 關鍵字type = TokenType.valueOf(value.toUpperCase());break;case 2:  // 標識符type = TokenType.IDENTIFIER;break;case 3:  // *type = TokenType.STAR;break;case 4:  // ,type = TokenType.COMMA;break;case 5:  // =type = TokenType.EQUALS;break;case 6:  // 字符串 (帶引號)type = TokenType.STRING;value = matcher.group(7);  // 去掉引號break;case 8:  // 數字type = TokenType.NUMBER;break;}if(type!=null){tokens.add(new Token(type, value));}pos = matcher.end();break;}}}tokens.add(new Token(TokenType.EOF, ""));return tokens;
}

}

4. AST 結構類 (SelectQuery.java)

import java.util.List;

public class SelectQuery {
public List columns;
public String table;
public Condition whereCondition;

public static class Condition {public String column;public String operator;public String value;
}

}

5. 語法解析器 (Parser.java)

import java.util.ArrayList;
import java.util.List;

public class Parser {
private final List tokens;
private int pos = 0;

public Parser(List<Token> tokens) {this.tokens = tokens;
}public SelectQuery parse() {SelectQuery query = new SelectQuery();parseSelect(query);parseFrom(query);parseWhere(query);return query;
}private void parseSelect(SelectQuery query) {consume(TokenType.SELECT);query.columns = new ArrayList<>();if (peek().type == TokenType.STAR) {consume(TokenType.STAR);query.columns.add("*");} else {do {query.columns.add(consume(TokenType.IDENTIFIER).value);} while (match(TokenType.COMMA));}
}private void parseFrom(SelectQuery query) {consume(TokenType.FROM);query.table = consume(TokenType.IDENTIFIER).value;
}private void parseWhere(SelectQuery query) {if (match(TokenType.WHERE)) {SelectQuery.Condition condition = new SelectQuery.Condition();condition.column = consume(TokenType.IDENTIFIER).value;consume(TokenType.EQUALS);Token valueToken = peek();if (valueToken.type == TokenType.STRING || valueToken.type == TokenType.NUMBER) {condition.value = valueToken.value;advance();} else {throw new RuntimeException("Expected string or number");}query.whereCondition = condition;}
}private Token consume(TokenType expected) {Token token = peek();if (token.type != expected) {throw new RuntimeException("Expected " + expected + ", found " + token.type);}advance();return token;
}private boolean match(TokenType type) {if (peek().type == type) {advance();return true;}return false;
}private Token peek() {return tokens.get(pos);
}private void advance() {pos++;
}

}

6. 測試主類 (Main.java)

import java.util.List;

public class Main {
public static void main(String[] args) {
String sql = “SELECT id, name FROM users WHERE age = 25”;
Lexer lexer = new Lexer(sql);
List tokens = lexer.tokenize();
Parser parser = new Parser(tokens);
SelectQuery query = parser.parse();

    System.out.println("Columns: " + query.columns);System.out.println("Table: " + query.table);if (query.whereCondition != null) {System.out.println("WHERE " + query.whereCondition.column + " = " + query.whereCondition.value);}
}

}

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代碼說明

1. 詞法分析器 (Lexer)

   • 使用正則表達式匹配 SQL 關鍵字、標識符、數字、字符串等 Token。
   • 跳過空白字符，返回 Token 列表。

2. 語法解析器 (Parser)

   • 遞歸下降解析器，依次解析 SELECT、FROM、WHERE 子句。
   • 構建 SelectQuery 對象存儲解析結果。

3. AST 結構 (SelectQuery)

   • 保存查詢的列、表名和過濾條件。

4. 測試示例 (Main)

   • 輸入 SQL 語句，輸出解析后的結構。

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運行結果

Columns: [id, name]
Table: users
WHERE age = 25

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支持特性

• 單表 SELECT 查詢
• 列名列表或 *
• 簡單的 WHERE 條件（僅支持 = 和字符串/數字值）

可根據需要擴展 WHERE 條件（如 >, <, AND/OR）和更復雜的數據類型。