MyBatis 如何使项目兼容多种数据库的解决方案

@Configuration //配置类
public class MyBatisConfig {
    @Bean
    public DatabaseIdProvider getDatabaseIdProvider() {
        DatabaseIdProvider databaseIdProvider = new VendorDatabaseIdProvider();
        Properties properties = new Properties();
        // Key值(即产品名)来源于数据库，需要提前查清楚 ，
        // value值(即databaseId)可以随便填，你填“1” "2" "3"也行，但建议有明确意义，像下面这样
        properties.setProperty("0racle", "oracle");
        properties.setProperty("MySQL", "mysql");
        properties.setProperty("DB2", "db2");
        properties.setProperty("Derby", "derby");
        properties.setProperty("H2", "h2");
        properties.setProperty("HSQL", "hsql");
        properties.setProperty("Informix", "informix");
        properties.setProperty("MS-SQL", "ms-sql");
        properties.setProperty("PostgresqL", "racle");
        properties.setProperty("sybase", "sybase");
        properties.setProperty("Hana", "hana");
        databaseIdProvider.setProperties(properties);
        return databaseIdProvider;
    }
}

SELECT * FROM table_name LIMIT offset, count;

SELECT * 
FROM (SELECT t.*, ROWNUM AS rn 
      FROM table_name t 
      WHERE ROWNUM <= offset + count) 
WHERE rn > offset;

SELECT NOW();

SELECT SYSDATE FROM DUAL;

INSERT INTO table_name (column1, column2) VALUES(value1, value2);
SELECT LAST_INSERT_ID();

INSERT INTO table_name (column1, column2) VALUES(seq.nextval, value2);
SELECT seq.currval from dual;

SELECT CAST('123' AS SIGNED) AS converted_value;  
-- 或者  
SELECT CONVERT('123', SIGNED) AS converted_value;

INSERT INTO table_name (column1, column2) VALUES(seq.nextval, value2);
SELECT seq.currval from dual;

SELECT CONCAT(column1, column2) FROM table_name;

SELECT column1 || column2 FROM table_name;

SELECT SUBSTRING('Hello World', 1, 5) AS substring_result;

SELECT SUBSTR('Hello World', 1, 5) AS substring_result FROM DUAL;

SELECT IFNULL(column1, "1") FROM table_name;

SELECT NVL(column1, "1") FROM table_name;

SELECT 'Hello World' REGEXP '^Hello' AS is_matched;

SELECT CASE WHEN REGEXP_LIKE('Hello World', '^Hello') THEN 'Matched' ELSE 'Not Matched' END AS is_matched FROM DUAL;

SELECT t1.*
FROM table_name t1
LEFT JOIN table_name t2
ON t1.column_name = t2.column_name AND t1.order_column > t2.order_column
WHERE t2.column_name IS NULL;

SELECT * 
FROM (SELECT *, ROW_NUMBER() OVER (PARTITION BY column_name ORDER BY order_column) as rn 
      FROM table_name) as t
WHERE rn = 1;

<select id = "getSysDateTime" databaseId="oracle">
	select
			TO_CHAR (sysdate, 'yyyyMMdd') sys_date,
			TO_CHAR (sysdate, 'HH24miss') sys_time
	from dual
</select>
<select id = "getSysDateTime" databaseId="mysql">
	select
			date_format (now(), '%Y%m%d') sys_date,
			date_format (now(), '%H%i%s') sys_time
	from dual
</select>

<select id = "getUserInfo" resultType = "UserInfo">
	select user_name, user_age
	from USERINFO
</select>

// SqlSessionFactoryBean
protected SqlSessionFactory buildSqlSessionFactory() throws Exception {
	// 省略无关代码
    if (this.databaseIdProvider != null) {
      try {
        targetConfiguration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
      } catch (SQLException e) {
        throw new NestedIOException("Failed getting a databaseId", e);
      }
    }
    // 省略无关代码
}

// XMLMapperBuilder
  private void buildStatementFromContext(List<XNode> list) {
    // 如果当前环境有DatabaseId，则以这个DatabaseId去加载对应的SQL
    if (configuration.getDatabaseId() != null) {
      buildStatementFromContext(list, configuration.getDatabaseId());
    }
    // 兜底，把某些没有指明DatabaseId的SQL加载进来
    buildStatementFromContext(list, null);
  }
  private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    for (XNode context : list) {
      final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
      try {
        statementParser.parseStatementNode();
      } catch (IncompleteElementException e) {
        configuration.addIncompleteStatement(statementParser);
      }
    }
  }

// XMLStatementBuilder
  public void parseStatementNode() {
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");
    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
      return;
    }
    // 省略无关代码
}
  private boolean databaseIdMatchesCurrent(String id, String databaseId, String requiredDatabaseId) {
    if (requiredDatabaseId != null) {
      return requiredDatabaseId.equals(databaseId);
    }
    if (databaseId != null) {
      return false;
    }
    id = builderAssistant.applyCurrentNamespace(id, false);
    if (!this.configuration.hasStatement(id, false)) {
      return true;
    }
    // skip this statement if there is a previous one with a not null databaseId
    MappedStatement previous = this.configuration.getMappedStatement(id, false); // issue #2
    return previous.getDatabaseId() == null;
  }

<select id = "getSysDateTime" databaseId="mysql">
	select
			date_format (now(), '%Y%m%d') sys_date,
			date_format (now(), '%H%i%s') sys_time
	from dual
</select>
<select id = "getSysDateTime">
	select
			TO_CHAR (sysdate, 'yyyyMMdd') sys_date,
			TO_CHAR (sysdate, 'HH24miss') sys_time
	from dual
</select>

    @Override
    @SuppressWarnings("unchecked")
    public V put(String key, V value) {
      if (containsKey(key)) {
        throw new IllegalArgumentException(name + " already contains value for " + key
            + (conflictMessageProducer == null ? "" : conflictMessageProducer.apply(super.get(key), value)));
      }
      if (key.contains(".")) {
        final String shortKey = getShortName(key);
        if (super.get(shortKey) == null) {
          super.put(shortKey, value);
        } else {
          super.put(shortKey, (V) new Ambiguity(shortKey));
        }
      }
      return super.put(key, value);
    }