I've done this when building the reporting side of a survey product.

The vast majority of your users won't use the complex filters.

Ask your customers what kinds of reports they want to build, usually it will be a series of AND requirements.

You can look at email filter builders for inspiration.

Usually it is something like building a series of :

Field (drop down) operator (drop down) value (text)

The tricky part in the ui is combining these (and/or) and if you really want to get fancy, supporting parena to change the precedence of the operators. A AND (b or c).

You must not convert this to SQL on the client side. You must validate the fields submitted are in your approved list on the backend (indexes/security).