1.什么是 JSQLParser？

1.什么是 JSQLParser？

JSQLParser 是一个开源的 Java 库，用于解析 SQL 语句并将其转换为抽象语法树（AST）。它支持多种 SQL 方言，包括 MySQL、PostgreSQL、Oracle 和 SQL Server 等。JSQLParser 使开发者能够轻松地分析、修改和生成 SQL 语句，广泛应用于数据库工具、ORM 框架和数据迁移工具等场景。

2.JSQLParser 的主要功能

SQL 解析：JSQLParser 能够将 SQL 查询字符串解析为结构化的对象模型，方便后续的操作和分析。

抽象语法树（AST）：解析后的 SQL 语句以 AST 的形式表示，开发者可以通过访问这些对象来获取 SQL 语句的各个组成部分，如选择字段、表名、条件等。

SQL 生成：除了解析，JSQLParser 还支持将 AST 重新生成 SQL 语句，这对于动态构建 SQL 查询非常有用。

支持多种 SQL 方言：JSQLParser 支持多种 SQL 方言，开发者可以根据需要选择合适的方言进行解析。

灵活的扩展性：JSQLParser 提供了丰富的 API，允许开发者根据具体需求扩展和定制解析器的行为。

3.JSQLParser 的使用场景

数据库工具：在数据库管理工具中，JSQLParser 可以用于解析用户输入的 SQL 查询，提供语法高亮、自动补全等功能。

ORM 框架：在对象关系映射（ORM）框架中，JSQLParser 可以帮助将对象模型转换为 SQL 查询，并解析 SQL 结果集。

数据迁移和转换：在数据迁移工具中，JSQLParser 可以解析源数据库的 SQL 语句，并生成目标数据库所需的 SQL 语句。

SQL 优化：通过解析 SQL 语句，开发者可以分析查询的性能，并进行优化。

4.如何使用 JSQLParser引入依赖：

在 Maven 项目中，可以通过以下依赖引入 JSQLParser：

dependency>
    groupId>com.github.jsqlparsergroupId>
    artifactId>jsqlparserartifactId>
    version>3.2version>
dependency>

解析 SQL 语句：

package com.et;
import net.sf.jsqlparser.parser.CCJSqlParserUtil;
import net.sf.jsqlparser.statement.Statement;
import net.sf.jsqlparser.statement.select.Select;
import net.sf.jsqlparser.statement.select.PlainSelect;
import net.sf.jsqlparser.statement.select.SelectItem;
import net.sf.jsqlparser.expression.Expression;
import java.util.List;
public class SqlParserExample {
    public static void main(String[] args) {
        // SQL query to be parsed
        String sql = "SELECT id, name FROM users WHERE age > 30";
        try {
            // Parse the SQL statement
            Statement statement = CCJSqlParserUtil.parse(sql);
            
            // Ensure the parsed statement is a SELECT statement
            if (statement instanceof Select) {
                Select selectStatement = (Select) statement;
                PlainSelect plainSelect = (PlainSelect) selectStatement.getSelectBody();
                // Get the selected columns
                List selectItems = plainSelect.getSelectItems();
                System.out.println("Selected columns:");
                for (SelectItem item : selectItems) {
                    System.out.println(item);
                }
                // Get the WHERE condition
                Expression where = plainSelect.getWhere();
                System.out.println("WHERE condition:");
                if (where != null) {
                    System.out.println(where);
                } else {
                    System.out.println("No WHERE condition");
                }
            }
        } catch (Exception e) {
            e.printStackTrace(); // Print the stack trace in case of an exception
        }
    }
}

Code Explanation

Package Declaration: The code is part of the com.et package.

Imports: Necessary classes from the JSQLParser library are imported to handle SQL parsing.

Main Class: The SqlParserExample class contains the main method, which is the entry point of the program.

SQL Query: A SQL query string is defined for parsing.

Parsing the SQL Statement: The SQL string is parsed using CCJSqlParserUtil.parse(sql).

Checking Statement Type: The code checks if the parsed statement is an instance of Select.

Getting Selected Columns: The selected columns are retrieved from the PlainSelect object and printed to the console.

Getting WHERE Condition: The WHERE condition is retrieved and printed. If there is no WHERE condition, a corresponding message is displayed.

Exception Handling: Any exceptions that occur during parsing are caught and printed to the console.

This code effectively demonstrates how to parse a SQL SELECT statement and extract the selected columns and WHERE conditions using JSQLParser.

生成 SQL 语句：

package com.et;
import net.sf.jsqlparser.parser.CCJSqlParserUtil;
import net.sf.jsqlparser.schema.Column;
import net.sf.jsqlparser.schema.Table;
import net.sf.jsqlparser.statement.select.Select;
import net.sf.jsqlparser.statement.select.SelectBody;
import net.sf.jsqlparser.statement.select.PlainSelect;
import net.sf.jsqlparser.statement.select.SelectItem;
import net.sf.jsqlparser.expression.LongValue;
import net.sf.jsqlparser.expression.BinaryExpression;
import net.sf.jsqlparser.expression.operators.relational.GreaterThan;
import net.sf.jsqlparser.statement.select.SelectExpressionItem; // Ensure SelectExpressionItem class is imported
import java.util.ArrayList;
import java.util.List;
public class SqlGeneratorExample {
    public static void main(String[] args) {
        // Create a Select object
        Select select = new Select();
        
        // Create a PlainSelect object
        PlainSelect plainSelect = new PlainSelect();
        
        // Set the selected columns
        List selectItems = new ArrayList();
        selectItems.add(new SelectExpressionItem(new Column("id"))); // Use Column class for "id"
        selectItems.add(new SelectExpressionItem(new Column("name"))); // Use Column class for "name"
        plainSelect.setSelectItems(selectItems);
        
        // Set the table
        Table table = new Table("users");
        plainSelect.setFromItem(table);
        
        // Set the WHERE condition
        BinaryExpression whereCondition = new GreaterThan(); // Create a GreaterThan expression
        whereCondition.setLeftExpression(new Column("id")); // Set the left expression to the "id" column
        whereCondition.setRightExpression(new LongValue(10)); // Set the right expression to a LongValue of 10
        plainSelect.setWhere(whereCondition);
        
        // Set the PlainSelect as the SelectBody
        select.setSelectBody(plainSelect);
        
        // Generate the SQL statement
        String generatedSql = select.toString();
        System.out.println(generatedSql); // Print the generated SQL statement
    }
}

Code Explanation

Package Declaration: The code is part of the com.et package.

Imports: Necessary classes from the JSQLParser library are imported to handle SQL generation.

Main Class: The SqlGeneratorExample class contains the main method, which is the entry point of the program.

Creating Select Object: A Select object is created to represent the SQL SELECT statement.

Creating PlainSelect Object: A PlainSelect object is created to define the details of the SELECT statement.

Setting Selected Columns: A list of SelectItem objects is created to hold the selected columns. Each column is added using the SelectExpressionItem class.

Setting Table: A Table object is created to specify the table from which to select data.

Setting WHERE Condition: A GreaterThan expression is created to define the WHERE condition. The left expression is set to the “id” column, and the right expression is set to a LongValue of 10.

Setting SelectBody: The PlainSelect object is set as the body of the Select statement.

Generating SQL Statement: The SQL statement is generated by calling toString() on the Select object, and the generated SQL is printed to the console.

以上只是一些关键代码，所有代码请参见下面代码仓库

代码仓库JSQLParser 的优缺点

优点：

缺点：

总结

JSQLParser 是一个强大的 SQL 解析工具，适用于各种 Java 应用程序。无论是数据库管理工具、ORM 框架还是数据迁移工具，JSQLParser 都能提供高效的 SQL 解析和生成能力。通过灵活的 API 和对多种 SQL 方言的支持，开发者可以轻松地处理 SQL 语句，提升开发效率。