MyBatis's foreach statement

The main use of foreach is in the build in condition , which can iterate a collection in an SQL statement. The properties of the Foreach element are mainly item,index,collection,open,separator,close.

Item represents the alias at which each element in the collection iterates.

index specifies a name that represents the position of each iteration during the iteration,

Open indicates what the statement starts with,

Separator indicates what symbol is used as a delimiter between each iteration.

Close indicates what ends with,

The most critical and error-prone thing when using foreach is the collection property, which must be specified, but in different cases the value of the property is not the same ,

There are 3 main cases:

1. If a single parameter is passed in and the parameter type is a list, thecollection property value is List

2. If a single parameter is passed in and the parameter type is an array, thevalue of the collection property is array

3. If the parameters passed in are multiple , we need to encapsulate them into a map , of course, the single parameter can also be encapsulated into a map, in fact, if you pass in the parameters, In breast it will be encapsulated into a map, themap key is the parameter name , so this time the collection property value is the incoming list or array object in its own encapsulated map key

Here's a look at the sample code for each of the three scenarios:

1. Type of single parameter list:

<select id= "Dynamicforeachtest" resulttype= "Blog" >

SELECT * from T_blog where ID in

<foreach collection= "list" index= "index" item= "Item" open= "(" separator= "," close= ")" >

#{item}

</foreach>

</select>

The value of the above collection is List, the corresponding mapper is the public list<blog> dynamicforeachtest (list<integer> IDs);

Test code:

@Test

public void Dynamicforeachtest () {

sqlsession session = Util.getsqlsessionfactory (). Opensession ();

Blogmapper blogmapper = Session.getmapper (Blogmapper.class); Annotation patterns are used here

list<integer> ids = new arraylist<integer> ();

Ids.add (1);

Ids.add (3);

Ids.add (6);

list<blog> blogs = blogmapper.dynamicforeachtest (IDS);

for (Blog blog:blogs)

SYSTEM.OUT.PRINTLN (blog);

Session.close ();

}

2. Type of single-parameter array arrays:

<select id= "Dynamicforeach2test" resulttype= "Blog" >

SELECT * from T_blog where ID in

<foreach collection= "array" index= "index" item= "Item" open= "(" separator= "," close= ")" >

#{item}

</foreach>

</select>

The above collection is an array, corresponding to the Mapper code: public list<blog> dynamicforeach2test (int[] IDs);

The corresponding test code:

@Test public void Dynamicforeach2test () {

sqlsession session = Util.getsqlsessionfactory (). Opensession ();

Blogmapper blogmapper = Session.getmapper (Blogmapper.class);

int[] ids = new int[] {1,3,6,9};

list<blog> blogs = blogmapper.dynamicforeach2test (IDS);

for (Blog blog:blogs)

SYSTEM.OUT.PRINTLN (blog);

Session.close ();

}

3. Encapsulate the parameters as a map type

<select id= "Dynamicforeach3test" resulttype= "Blog" >

SELECT * from T_blog where title like "%" ${title} "%" and ID in

<foreach collection= "IDs" index= "index" item= "Item" open= "(" separator= "," close= ")" >

#{item}

</foreach>

</select> the value of the above collection is IDs, which is the key of the incoming parameter map, corresponding to the Mapper code:

Public list<blog> dynamicforeach3test (map<string, object> params);

Corresponding Test code:

@Test

public void Dynamicforeach3test () {

sqlsession session = Util.getsqlsessionfactory (). Opensession ();

Blogmapper blogmapper = Session.getmapper (Blogmapper.class);

final List<integer> ids = new arraylist<integer> ();

Ids.add (1); Ids.add (2);

Ids.add (3); Ids.add (6);

Ids.add (7); Ids.add (9);

map<string, object> params = new hashmap<string, object> ();

Params.         put("IDs", IDs); Params.put ("title", "China");

list<blog> blogs = blogmapper.dynamicforeach3test (params);

for (Blog blog:blogs)

SYSTEM.OUT.PRINTLN (blog);

Session.close ();

}

MyBatis's foreach statement