SQL

SQL is something that's gonna be there until the end of the world. Even with the rise of Big Data Systems, relational database systems have not lost importance. Also, RDBMS companies are rolling out Big Data flavored features like scaling, distributed processing, data warehousing to handle large-scale structured data with the current implementations of Relational Databases. So the point is SQL is a very good place to start with and it has a great future. But I think you need to polish your skills and widen your scope to accommodate different requirements. So you should keep these things in your mind:
1. Start with a platform. SQL is an ANSI standard database scripting language, but that has been customized by large technology corps like Microsoft, Oracle, IBM, Sybase and so on. Pick up a platform and learn SQL on it. So if it is Microsoft, then install SQL Server and start exploring T-SQL. General statements are common on all platforms to some extent, but as you go deep, there changes at query, datatypes, execution levels
2. Start exploring more about the database engine of the database product. To write better queries, you need to know how they are evaluated on the server when you fire them.
3. Get some grip on database administration.
4. Having knowledge of reporting and data integration would be a plus point for you. So if it is SQL Server then try your hands on SSIS (SQL Server Integration Services) and SSRS (SQL Server Reporting Services).
5. As a last resort, if you are going to give a dive into Big Data, then learn HIVE. It is similar to SQL.

I would love to share few books name:-
SQL Practice Problems: 57 beginning, intermediate, and advanced challenges for you to solve using a 'learn-by-doing' approach:- The ability to work with data and write SQL is currently one of the most in-demand job skills.
T-Sql Querying:- Squeeze maximum performance and efficiency from every T-SQL query you write or tune. Four leading experts take an in-depth look at T-SQLs internal architecture and offer advanced practical techniques for optimizing response time and resource usage.
MYSQL: THE COMPLETE REFERENCE:- Get comprehensive coverage of all the powerful new features of MySQL, one of the fastest-and free-relational databases in use today.
Oracle Database 12C Hands-on SQL and PL/SQL:- The book teaches the basics of the Oracle database from a beginners perspective to the advanced concepts using a hands-on approach. Each and every concept has been elaborated with suitable practical examples along with code for clear and precise understanding of the topic.