MyBatis设计SQL返回布尔值(Boolean)的常见方法

<select id="checkUserExists" resultType="_boolean">
    SELECT COUNT(*) > 0 
    FROM users 
    WHERE username = #{username}
</select>

<select id="isAccountActive" resultType="java.lang.Boolean">
    SELECT (status = 'ACTIVE') 
    FROM accounts 
    WHERE account_id = #{id}
</select>

<select id="hasUserPermission" resultType="boolean">
    SELECT EXISTS(
        SELECT 1 
        FROM permissions 
        WHERE user_id = #{userId} 
        AND permission = #{perm}
    )
</select>

<update id="deactivateUser">
    UPDATE users SET active = 0 WHERE id = #{id}
</update>

// 返回Boolean类型
Boolean deactivateUser(Long id);

public class BooleanTypeHandler extends BaseTypeHandler<Boolean> {
    @Override
    public void setNonNullParameter(...) {
        ps.setString(i, parameter ? "Y" : "N");
    }
    
    @Override
    public Boolean getNullableResult(ResultSet rs, String col) {
        return "Y".equals(rs.getString(col));
    }
    // 其他重写方法省略...
}

<resultMap type="User">
    <result property="vip" column="is_vip" 
            typeHandler="com.example.BooleanTypeHandler"/>
</resultMap>

SELECT COUNT(*) > 0 FROM ...  -- 简单条件
SELECT EXISTS(...)           -- 复杂条件

<!-- 错误：resultType不能是boolean -->
<select id="isAdmin" resultType="boolean">
     SELECT is_admin FROM users WHERE id=#{id}
</select>

<select id="isAdmin" resultType="_boolean">
     SELECT IF(is_admin='Y', true, false) 
     FROM users 
     WHERE id=#{id}
</select>

public interface UserMapper {
    // 检查用户名是否存在
    boolean existsByUsername(@Param("name") String username);
    
    // 禁用用户并返回是否成功
    Boolean deactivateUser(Long id);
}

<select id="existsByUsername" resultType="_boolean">
    SELECT EXISTS(
        SELECT 1 FROM users 
        WHERE username = #{name}
    )
</select>
 
<update id="deactivateUser">
    UPDATE users SET active = 0 WHERE id = #{id}
</update>

UserMapper mapper = session.getMapper(UserMapper.class);
 
// 检查用户名
boolean exists = mapper.existsByUsername("john_doe");
System.out.println("Username exists? " + exists);
 
// 禁用用户
Boolean success = mapper.deactivateUser(1001L);
if(success) {
    System.out.println("User deactivated");
}