Do not use Left-join, such as multi-table association query, only with single-table query and Java program, easy to achieve "multi-table query" effect

The last time we mentioned that not using the Left-loin Association query, it might be to improve efficiency or to configure caching, or to simplify the writing of SQL statements. Just write a single-table query, SQL is really too simple. The problem is that querying data for multiple tables is still very much needed.

So there is a strong need to query 2 collections, how to combine data from 2 collections into 1 sets, and data associations to be correct. In practice, I refine the following process and tool methods:

Process
Query the 1th collection first, query the 2nd collection according to the result of the 1th collection, merge 2 sets
Public Pagevo listpage (Pagevo form) {
list<map<string, object>> list = transferdao.listpage (form);
if (list = null && list.size () >0) {
list<map<string, object>> loanlist = loaninfoservice.batchlist (list);
Mergeutilmerge (list, loanlist);
Form.setlist (list);
}
return form;
}

tool methods for merging collections
/**
* Merges the associated data collection into the main table collection, merges the data from the 2 collections into a single collection, and associates them with a key to replace the database's left-join and other associated queries.
*
* methods are divided into 3 classes: Merging 2 elements for the collection of maps, merging 2 maps, finding 1 maps in another element for the associated map of the set.
*
* Optional Parameters: 2 Map Association key may be 1, or 2; When merging 2 maps, you can prefix the key of the associated map with prefix, prevent duplicate names, or do not add them to each other.
*
* Not applicable to the scene: Multi-table Query search and query conditions in the association table and the need for paging, only the use of single-table query +merge tool, is not enough.
* @author [email protected] November 26, 2014
*/
public class Mergeutil {
Merges all of the map/////////////////////in a 2 collection

/**
* Merge the associated data collection into the primary table, combine the data from 2 sets into a single collection, and associate it with a primary key (without a database left join)
*/
Merge 2 sets, find the associated map first, and then merge the 2 separate map,2 map with the associated key is the same
public static void Merge (List<map<string, object>> mainlist,
List<map<string, object>> relationlist, String Relationkey,
String prefix);

//merges 2 sets, finds the associated map first, merges 2 separate maps, merges the key of the map with the prefix, and the associated key in the 2 map is 2
public static void merge (list<map<string, object>> mainlist,
  String MainKey , list<map<string, object>> relationlist,
  String RelationKey, String prefix);

//merges 2 sets, finds the associated map first, and merges 2 separate map,2 map with the associated key is the same as 1
public static void merge (List <map<string, object>> mainlist,
  List<Map<String, Object>> relationlist, String relationkey);

Merges 2 sets, and the map in 2 sets is associated with a key
public static void Merge (List<map<string, object>> mainlist,
String Mainkey, list<map<string, object>> relationlist,
String Relationkey);

Merge 2 map, prefix prefix can not, not the case, key name, will overwrite/////////////////////

Merge 2 map,relation will be put in main, key all plus prefix
public static void Merge (Map<string, object> Mainmap,
Map<string, object> relation, String prefix);

Merges 2 maps, and if there are duplicates, the Relationmap overrides the one in main
public static void Merge (Map<string, object> Mainmap,
Map<string, object> Relationmap);

/////////////////////////look for a map in a set of associations map/////////////////////
/**
 * the target object from a collection, and if not found, returns NULL
 * 
 * @param 2 map common key
 */
public static map<string, object> Relationmap (map<string, object> mainmap,
  list<map<string, object>> List, String relationkey);

Found 1 map objects that need to be associated in the collection list, specifying their own key
public static map<string, object> Relationmap (map<string, object> Mainmap,
String Mainkey, list<map<string, object>> List, string relationkey);

The source code of the tool method is not provided, and the process and base library of Java web development are still accumulating continuously.

Original starting: http://fansunion.cn/article/detail/540.html

Do not use Left-join, such as multi-table association query, only with single-table query and Java program, easy to achieve "multi-table query" effect