For all of those new to SQL in our DBA team, I recommend two books for them to practice on.

 The first is immediate and short-to-medium term. It is the SQL Cookbook from Safari [1]. It starts off very simply, sets a task to be solved, gives answers for 4 RDBMS and explains each solution. I found it to be the best way to learn SQL. It is especially good if you have, say, an hour's commute on a train each morning. A laptop is nice but not at all necessary.

 The second is the medium-to-long term. It is the book to prepare you for the base MCP exam (in this case 70-461) [2]. I tell the new new juniors to give the book a year and it will bring them up to a high level. There is a lot within that they don't really need to know, but they need to know that it exists and what it can do (XML, Full-Text search and so on). If they apply themselves, they should be able to get an MCP exam within a sitting or two. MCPs are good to have on CVs.

We have the books in the office and they are useful when times are not so busy.

 Aside from that, there is the usual explaining of how things are done and the introduction of new techniques as the oportunity arises.

[1] https://www.safaribooksonline.com/library/view/sql-cookbook/0596009763/
[2] https://www.microsoft.com/en-gb/learning/exam-70-461.aspx