learn SQL

SQL is short for Structured Query Language and is a widely used database language, providing means of data manipulation (store, retrieve, update, delete) and database creation.

Almost all modern Relational Database Management Systems like MS SQL Server, Microsoft Access, MSDE, Oracle, DB2, Sybase, MySQL, Postgres and Informix use SQL as standard database language. Now a word of warning here, although all those RDBMS use SQL, they use different SQL dialects. For example MS SQL Server specific version of the SQL is called T-SQL, Oracle version of SQL is called PL/SQL, MS Access version of SQL is called JET SQL, etc.

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That is not entirely correct. Oracle has its version of SQL. SQL is not PL/SQL, and PL/SQL is not SQL. You can invoke PL/SQL from SQL, and you can embed SQL inside of PL/SQL.