As previously noted, learning to write SQL is simple enough. You can use online resources such as W3Schools, youtube or MS virtual academy if you're simply looking to get a grounding for working with SQL.
Do you have put much thought into which implementation of SQL you wish to learn about? I.e. MySQL, Oracle, MSSQL etc?
The part that a lot of people seem to trip up on is that they pick up the SQL syntax, but then neglect to fully understand how the underlying engines work, and then find that their queries may perform poorly, or experience temperamental issues. That said, it's totally unrealistic to expect everybody who works with SQL to be of a certified master of the the system.
In terms of MSSQL, anything written, or co-authored by Itzik Ben-gan tend to be superb for gaining a solid foundation, but are not necessarily geared for someone with 0 prior knowledge. The problem with a lot of programming related books is that they tend to be quite a dry read, and this doubly applies for database related materials. From what limited experience I have of Oracle, the online documentation on their website does seem to be of a reasonable quality.