Mongodb left join query the reason why as is empty

problem description

I have two tables, the user table and the dynamic table. I want to query all the dynamic users while traversing to get the corresponding users
all I use the left join, but the as returned by the query is empty. Why

the environmental background of the problems and what methods you have tried

this is my table
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mongodb compass users s objectId user sex notecontent id

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Apr.11,2022
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b38b4b-2c129.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b38b4b-2c129.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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