这里做了比较清晰的解释: http://mybatis.github.io/mybatis-3/java-api.html

SqlSession

As mentioned above, the SqlSession instance is the most powerful class in MyBatis. It is where you’ll find all of the methods to execute statements, commit or rollback transactions and acquire mapper instances.

There are over twenty methods on the SqlSession class, so let’s break them up into more digestible groupings.

Statement Execution Methods
These methods are used to execute SELECT, INSERT, UPDATE and DELETE statements that are defined in your SQL Mapping XML files. They are pretty self explanatory, each takes the ID of the statement and the Parameter Object, which can be a primitive (auto-boxed or wrapper), a JavaBean, a POJO or a Map.

<T> T selectOne(String statement, Object parameter)
<E> List<E> selectList(String statement, Object parameter)
<K,V> Map<K,V> selectMap(String statement, Object parameter, String mapKey)
int insert(String statement, Object parameter)
int update(String statement, Object parameter)
int delete(String statement, Object parameter)

The difference between selectOne and selectList is only in that selectOne must return exactly one object or null (none). If any more than one, an exception will be thrown. If you don’t’ know how many objects are expected, use selectList. If you want to check for the existence of an object, you’re better off returning a count (0 or 1). The selectMap is a special case in that it is designed to convert a list of results into a Map based on one of the properties in the resulting objects. Because not all statements require a parameter, these methods are overloaded with versions that do not require the parameter object.

The value returned by the insert, update and delete methods indicate the number of rows affected by the statement.

<T> T selectOne(String statement)
<E> List<E> selectList(String statement)
<K,V> Map<K,V> selectMap(String statement, String mapKey)
int insert(String statement)
int update(String statement)
int delete(String statement)

而在mapper的xml中, 不需要(也不能)给insert, update, delete指定resultType, 如这里所解释

http://mybatis.github.io/mybatis-3/sqlmap-xml.html

insert, update, delete只支持这些Attributes: id, parameterType, parameterMap, flushCache, timeout, statementType, useGeneratedKeys, keyProperty, keyColumn, databaseId