最近有兼容多数据库的需求，原有数据库使用的mysql，现在需要同时兼容mysql和pgsql，后期可能会兼容更多。
mysql和pgsql很多语法和函数不同，所以有些sql需要写两份，于是在全网搜索如何在mapper中sql不通用的情况下兼容多数据库，中文网络下，能搜到的解决方案大概有两种：1.使用@DS注解的动态数据源；2.使用数据库厂商标识，即databaseIdProvider。第一种多用来同时连接多个数据源，且配置复杂，暂不考虑。第二种明显符合需求，只需要指定sql对应的数据库即可，不指定的即为通用sql。
常规方法
在全网搜索databaseIdProvider的使用方法，大概有两种：
1.在mybatis的xml中配置，大多数人都能搜到这个结果：
<databaseIdProvider type="DB_VENDOR"><property name="MySQL" value="mysql"/><property name="Oracle" value="oracle" />
</databaseIdProvider>然后在mapper中：
<select id="selectStudent" databaseId="mysql">select * from student where name = #{name} limit 1
</select>
<select id="selectStudent" databaseId="oracle">select * from student where name = #{name} and rownum < 2
</select>2.创建mybatis的配置类：
import org.apache.ibatis.mapping.DatabaseIdProvider;
import org.apache.ibatis.mapping.VendorDatabaseIdProvider;
import org.apache.ibatis.session.SqlSessionFactory;
import org.mybatis.spring.SqlSessionFactoryBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.io.support.PathMatchingResourcePatternResolver;import javax.sql.DataSource;
import java.util.Properties;@Configuration
public class MyBatisConfig {@Beanpublic DatabaseIdProvider databaseIdProvider() {VendorDatabaseIdProvider provider = new VendorDatabaseIdProvider();Properties props = new Properties();props.setProperty("Oracle", "oracle");props.setProperty("MySQL", "mysql");props.setProperty("PostgreSQL", "postgresql");props.setProperty("DB2", "db2");props.setProperty("SQL Server", "sqlserver");provider.setProperties(props);return provider;}@Beanpublic SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {SqlSessionFactoryBean factoryBean = new SqlSessionFactoryBean();factoryBean.setDataSource(dataSource);factoryBean.setMapperLocations(new PathMatchingResourcePatternResolver().getResources("classpath*:mapper/*Mapper.xml"));factoryBean.setDatabaseIdProvider(databaseIdProvider());return factoryBean.getObject();}
}这两种方法，包括在mybatis的github和官方文档的说明，都是看得一头雾水，因为前后无因果关系，DB_VENDOR这种约定好的字段也显得很奇怪，为什么要配置DB_VENDOR？为什么mysql需要写键值对？键值对的key是从那里来的？全网都没有太清晰的说明。
一些发现
有没有更简单的办法？
mybatis的入口是SqlSessionFactory，如果要了解mybatis的运行原理，从这个类入手是最合适的，于是顺藤摸瓜找到了SqlSessionFactoryBuilder类，这个类有很多build方法，打断点之后发现当前配置走的是
public SqlSessionFactory build(Configuration config) {return new DefaultSqlSessionFactory(config);}这个Configuration类就非常显眼了，点进去之后发现这个类的成员变量就是可以在application.yml里直接设置值的变量
public class Configuration {protected Environment environment;protected boolean safeRowBoundsEnabled;protected boolean safeResultHandlerEnabled;protected boolean mapUnderscoreToCamelCase;protected boolean aggressiveLazyLoading;protected boolean multipleResultSetsEnabled;protected boolean useGeneratedKeys;protected boolean useColumnLabel;protected boolean cacheEnabled;protected boolean callSettersOnNulls;protected boolean useActualParamName;protected boolean returnInstanceForEmptyRow;protected String logPrefix;protected Class<? extends Log> logImpl;protected Class<? extends VFS> vfsImpl;protected LocalCacheScope localCacheScope;protected JdbcType jdbcTypeForNull;protected Set<String> lazyLoadTriggerMethods;protected Integer defaultStatementTimeout;protected Integer defaultFetchSize;protected ResultSetType defaultResultSetType;protected ExecutorType defaultExecutorType;protected AutoMappingBehavior autoMappingBehavior;protected AutoMappingUnknownColumnBehavior autoMappingUnknownColumnBehavior;protected Properties variables;protected ReflectorFactory reflectorFactory;protected ObjectFactory objectFactory;protected ObjectWrapperFactory objectWrapperFactory;protected boolean lazyLoadingEnabled;protected ProxyFactory proxyFactory;protected String databaseId;protected Class<?> configurationFactory;protected final MapperRegistry mapperRegistry;protected final InterceptorChain interceptorChain;protected final TypeHandlerRegistry typeHandlerRegistry;protected final TypeAliasRegistry typeAliasRegistry;protected final LanguageDriverRegistry languageRegistry;protected final Map<String, MappedStatement> mappedStatements;protected final Map<String, Cache> caches;protected final Map<String, ResultMap> resultMaps;protected final Map<String, ParameterMap> parameterMaps;protected final Map<String, KeyGenerator> keyGenerators;protected final Set<String> loadedResources;protected final Map<String, XNode> sqlFragments;protected final Collection<XMLStatementBuilder> incompleteStatements;protected final Collection<CacheRefResolver> incompleteCacheRefs;protected final Collection<ResultMapResolver> incompleteResultMaps;protected final Collection<MethodResolver> incompleteMethods;protected final Map<String, String> cacheRefMap;
……这里面的配置有些非常眼熟，比如logImpl，可以使用mybatis.configuration.log-impl直接设置值，那么同理，databaseId是不是也可以使用mybatis.configuration.databaseId设置值？答案是肯定的，而且这样设置值，绕过了databaseIdProvider也可以生效。
最简单的方法
如果你的springboot偏向使用application.yml配置或者使用了spring cloud config，又要兼容多数据库，那么你可以加一条配置
mybatis.configuration.database-id: mysql
或者
mybatis.configuration.database-id: orcale然后在你的mapper中
<select id="selectStudent" databaseId="mysql">select * from student where name = #{name} limit 1
</select>
<select id="selectStudent" databaseId="oracle">select * from student where name = #{name} and rownum < 2
</select>
或者
<select id="selectStudent">select * from student where <if test="_databaseId=='mysql'">name = #{name} limit 1</if><if test="_databaseId=='oracle'">name = #{name} and rownum < 2</if>
</select>
即可切换数据库，不影响其他任何配置，而且也不用纠结databaseIdProvider里的key应该怎么填写了。
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