MyBatis 中有哪些动态 SQL 标签？它们的作用分别是什么？如何实现的？

骑猪看日落 2022-12-13 11:29 178阅读 0赞

9 种动态 SQL 标签：if、choose、when、otherwise、trim、where、set、foreach、bind

1 种注解中使用动态 SQL 标签：script

*  if: 根据条件判断
 *  choose、when、otherwise: 组合使用，选择多个条件中的一个
 *  where: where 元素只会在子元素返回任何内容的情况下才插入 “WHERE” 子句，若子句的开头为 “AND” 或 “OR”，where 元素也会将它们去除
 *  trim: 定制类似 where 标签的功能
 *  set: 用于动态包含需要更新的列，忽略其它不更新的列
 *  foreach: 对集合进行遍历
 *  bind: 允许你在 OGNL 表达式以外创建一个变量，并将其绑定到当前的上下文
 *  script: 要在带注解的映射器接口类中使用动态 SQL，可以使用 script 元素

官方说明文档：

[https://mybatis.org/mybatis-3/zh/dynamic-sql.html][https_mybatis.org_mybatis-3_zh_dynamic-sql.html]

源码实现的入口在这里 XMLScriptBuilder 类中

protected MixedSqlNode parseDynamicTags(XNode node) {
        List< SqlNode> contents = new ArrayList< >();
        NodeList children = node.getNode().getChildNodes();
        for (int i = 0; i < children.getLength(); i++) {
          XNode child = node.newXNode(children.item(i));
          if (child.getNode().getNodeType() == Node.CDATA_SECTION_NODE || child.getNode().getNodeType() == Node.TEXT_NODE) {
            ...
          } else if (child.getNode().getNodeType() == Node.ELEMENT_NODE) { // issue #628
            //根据 node 名称获取对应 handler
            String nodeName = child.getNode().getNodeName();
            NodeHandler handler = nodeHandlerMap.get(nodeName);
            if (handler == null) {
              throw new BuilderException("Unknown element <" + nodeName + "> in SQL statement.");
            }
            handler.handleNode(child, contents);
            isDynamic = true;
          }
        }
        return new MixedSqlNode(contents);
      }

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[https_mybatis.org_mybatis-3_zh_dynamic-sql.html]: https://mybatis.org/mybatis-3/zh/dynamic-sql.html
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