How to nest lqip inside mainImage object in Sanity GROQ query?

Yes, absolutely! You can achieve this using the GROQ spread operator (...) to merge the lqip property into the mainImage object. Here's how:

*[_type == "post"][0] {
  "mainImage": {
    ...mainImage,
    "lqip": mainImage.asset->metadata.lqip
  }
}

This works by:

1. Creating a new mainImage object in the projection
2. Using ...mainImage to spread all the existing properties from the original mainImage field (including _type, altText, and asset)
3. Adding the lqip property by dereferencing the asset metadata

The result will be exactly what you're looking for:

{
  "mainImage": {
    "_type": "image",
    "altText": "Mountains with pink sky and turquoise water",
    "asset": {
      "_ref": "image-db89f238f31f609d2cac163248de2105d4c1f06c-3374x2000-jpg",
      "_type": "reference"
    },
    "lqip": "data:image/jpeg;base64,/9j/2wBDAAYEBQYFBAYGBQYHBw..."
  }
}

The ellipsis/spread operator (...) in GROQ is perfect for this kind of object composition where you want to keep all existing fields while adding new ones. Just note that the spread operator is always evaluated first regardless of where it appears in the projection, so any properties you define after it will override properties with the same name from the spread.