Corporate culture is a drinking culture. Boozy business lunches and dinners abound. Not to mention the "networking" cocktail parties and receptions. When and how much to drink seems to be some unspoken code that corporate types just know. This article puts some of those rules in writing.
Rule #1 Follow your host. If the person who invited you orders a drink, it is safe to have one yourself. In a client setting, wait to see if the client orders a drink. If the client is drinking, order a drink other than water so that the client is not drinking alone. This drink can be alcoholic or non-alcoholic. Joining in an alcoholic beverage with the client may be considered more polite in some situations, but courtesy drinking is ill-advised if it would result in some other embarrassment.
Rule #2 Don't order liquor at lunch if you have to go back to work. A single glass of wine with lunch is advisable if you drink at lunch at all.