mybatis-plus一对多关联查询踩坑

环境

1.sql server 数据库
2.使用mybatis-plus分页插件

需求

有两种表分别是电脑表、电脑配套表。两张表的关系是一对多。表数据如下：

电脑表

电脑配套表

需求是查询那些电脑拥有特殊配套。

正常查询

image.png

过滤只返回电脑信息

image.png

mapper xml的代码

    
        select DISTINCT c.* from computer c LEFT join computer_part cp on c.id = cp.computer_id
        where cp.type ='特殊配套'
    

swagger调用返回结果

swagger

很简单的功能可以看到total是2,但是数据内容是4。分别有一条重复。

原因在于上面的正常查询，确实是4条数据。但是mapper里面明明已经配置了distinct为什么还会重复呢？

因为使用了mybatis-plus分页插件，所以实际查询语句是被起包装过的，具体，通过控制台可以找到实际sql执行语句。如下：

image.png

WITH selectTemp AS (SELECT DISTINCT TOP 100 PERCENT ROW_NUMBER() OVER (ORDER BY CURRENT_TIMESTAMP) as __row_number__, c.* from computer c LEFT join computer_part cp on c.id = cp.computer_id where cp.type ='特殊配套') SELECT * FROM selectTemp WHERE __row_number__ BETWEEN 1 AND 10 ORDER BY __row_number__

sql语句拷贝到数据库连接工具执行：

sql执行结果

确实是四条。
到此真相大白，确实是插件的锅。查看插件源码：

public String buildPaginationSql(String originalSql, long offset, long limit) {
        StringBuilder pagingBuilder = new StringBuilder();
        String orderby = getOrderByPart(originalSql);
        String distinctStr = "";
        String loweredString = originalSql.toLowerCase();
        String sqlPartString = originalSql;
        if (loweredString.trim().startsWith("select")) {
            int index = 6;
            if (loweredString.startsWith("select distinct")) {
                distinctStr = "DISTINCT ";
                index = 15;
            }

            sqlPartString = originalSql.substring(index);
        }

        pagingBuilder.append(sqlPartString);
        if (StringUtils.isEmpty(orderby)) {
            orderby = "ORDER BY CURRENT_TIMESTAMP";
        }

        StringBuilder sql = new StringBuilder();
        sql.append("WITH selectTemp AS (SELECT ").append(distinctStr).append("TOP 100 PERCENT ").append(" ROW_NUMBER() OVER (").append(orderby).append(") as __row_number__, ").append(pagingBuilder).append(") SELECT * FROM selectTemp WHERE __row_number__ BETWEEN ").append(offset + 1L).append(" AND ").append(offset + limit).append(" ORDER BY __row_number__");
        return sql.toString();
    }

可以看到它确实是把我自带的distinct给提出去，再跟ROW_NUMBER函数拼接，导致去重失败。也可以看到源码中select distinct要是不只间隔一个空格，它还识别不出来。经过尝试多个空格还是导致报错。到此，基本锅已定。我这个版本是3.0.3。github上最新的是3.4.0。源码如下

 public DialectModel buildPaginationSql(String originalSql, long offset, long limit) {
        StringBuilder pagingBuilder = new StringBuilder();
        String orderby = getOrderByPart(originalSql);
        String distinctStr = StringPool.EMPTY;

        String loweredString = originalSql.toLowerCase();
        String sqlPartString = originalSql;
        if (loweredString.trim().startsWith("select")) {
            int index = 6;
            if (loweredString.startsWith("select distinct")) {
                distinctStr = "DISTINCT ";
                index = 15;
            }
            sqlPartString = sqlPartString.substring(index);
        }
        pagingBuilder.append(sqlPartString);

        // if no ORDER BY is specified use fake ORDER BY field to avoid errors
        if (StringUtils.isBlank(orderby)) {
            orderby = "ORDER BY CURRENT_TIMESTAMP";
        }
        long firstParam = offset + 1;
        long secondParam = offset + limit;
        String sql = "WITH selectTemp AS (SELECT " + distinctStr + "TOP 100 PERCENT " +
            " ROW_NUMBER() OVER (" + orderby + ") as __row_number__, " + pagingBuilder +
            ") SELECT * FROM selectTemp WHERE __row_number__ BETWEEN " +
            //FIX#299：原因：mysql中limit 10(offset,size) 是从第10开始（不包含10）,；而这里用的BETWEEN是两边都包含，所以改为offset+1
            firstParam + " AND " + secondParam + " ORDER BY __row_number__";
        return new DialectModel(sql);
    }

一看终究还是错付了。实际尝试还是会存在同样的问题。
我暂时没想通为啥作者要先处理 select 或者select distinct，而不是直接把原始sql整段保留，然后在拼接ROW_NUMBER函数进行分页处理。

解决

1.等待作者处理
2.手动分页
3.子查询的方式，避免连表查询
最终处理方案3，修改mapper代码如下：

    
        select  * from computer c
        where id in (select computer_id from computer_part where type = '特殊配套')
    

swagger执行结果

期望的结果

结语

本文到此结束。
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我也是mybatis-plus的小白使用者，本文如有不正确之处，望各种留情。