There are four markets that come up in the guidebooks for Bologna. Naturally, all the guides tell you how great they are and I'm going to tell you where to not waste your time.
Mercato di Mezzo (recommended)
Probably not by accident, the Eataly, Bologna is next door and all access from Eataly directly to the Mercato is closed. The Mercato is a series of independent food stalls serving various Italian items. Cool place to be for apertivo. Plenty of wine selections and food to eat. Lots of action and vibe in the place.
In the area around Mercato di Mezzo, there are plenty of choices for apertivo and a wide variety of things to eat. Yes, there are plenty of places to get mortadella and prosciutto, but there are other casual places serving their take on the apertivo time, both inside and outside on the street.
Fries with Eyes, also known as anchovies were being served as a special at the fish vendor. Along with some apertivo items served on bread, soft cooked pumpkin, wilted greens, and some cheese thing on bread, along with cold lasagna, for 8eu per glass of wine, one could eat their entire meal without technically purchasing food, only drinks.
Tortilloni with pumpkin in a sage butter sauce and topped with parmigiano was another winner and at 8.50eu for the plate
Mercato di Erbe, no pictures, forgot to charge phone (recommended).
Cool hangout for lunch, either sit down or quick take and eat. I had a typical dish of pasta with sausage in a light cream sauce. My friend had fried chicken and fries. Both were great. If anything, getting a quick fix of chicken fingers was the highlight, 10x better than a cotaletta sandwich, those suck. The dessert was a yogurt cream spiked with honey and something else. Thick and rich and slightly sweet, but no way it was all yogurt, it had to have either sour cream/creme fresh/whipping cream in it somehow.
Mercato di Pizzaolo (not recommended)...is the Ramate, not a market of food stalls. If you need a cheap scarf, hat, bag, etc. come here on a weekend. Otherwise, don't bother.
Mercati della Terra (not recommended in Winter) isn’t that big of a deal in winter. It may be the coolest market in Spring and Summer, but in the Winter, it’s just ok. There is an adjacent open lot with food vendors and beer, which could get lively.