How to perform joins on non-primary columns and complex select statements in loopback 4?

Describe the bug

Hi i am new to loopback and need to create an API with business logic that contains several complex queries. I cant seem to find a way to join two tables with non-primary columns. For example, select * from e_c left join e on e.ed=e_c.ed Table relations: e_c.ed fk to ed table on id e.ed fk to ed table on id

On the same note writing complex select statements are also needed. For example, select (case when completed_on is null then 1 else 0 end) flag from e or select sum(rating) from f_b

Kindly provide some method by which these can be achieved via loopback 4

Logs

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Additional information

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Reproduction

未指定源文件、测试或入口点。首先参考非主列连接和复杂 SELECT 示例，检查 LoopBack 4 的关系和查询 API；完成的标准是确定一种有文档说明的受支持方法，或明确记录限制和所需范围。