试了半天，原来通过
db.taobaoItem.find({"coupon_price":{$ne:null}}
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MongoDB Operators
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Query Selectors
Comparison
| $all | Matches arrays that contain all elements specified in the query. |
| $gt | Matches values that are greater than the value specified in the query. |
| $gte | Matches values that are equal to or greater than the value specified in the query. |
| $in | Matches any of the values that exist in an array specified in the query. |
| $lt | Matches values that are less than the value specified in the query. |
| $lte | Matches values that are less than or equal to the value specified in the query. |
| $ne | Matches all values that are not equal to the value specified in the query. |
| $nin | Matches values that do not exist in an array specified to the query. |
Logical
| $or | Joins query clauses with a logical OR returns all documents that match the conditions of either clause. |
| $and | Joins query clauses with a logical AND returns all documents that match the conditions of both clauses. |
| $not | Inverts the effect of a query expression and returns documents that do not match the query expression. |
| $nor | Joins query clauses with a logical NOR returns all documents that fail to match both clauses. |
Element
| $exists | Matches documents that have the specified field. |
| $mod | Performs a modulo operation on the value of a field and selects documents with a specified result. |
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