code: public String toSQL(final MasterSlaveRule masterSlaveRule, final ShardingDataSourceMetaData shardingDataSourceMetaData) {
          StringBuilder result = new StringBuilder();
          for (Object each : segments) {
              if (each instanceof SchemaPlaceholder) {
                  result.append(shardingDataSourceMetaData.getActualDataSourceMetaData(masterSlaveRule.getMasterDataSourceName()).getSchemaName());
              } else {
                  result.append(each);
              }
          }
          return result.toString();
      }
docs: Convert to SQL unit.
  
  @param masterSlaveRule master slave rule
  @param shardingDataSourceMetaData sharding data source meta data
  @return SQL