HTSQL a high-level query language and Web service gateway for relational databases. It comes with source code and is royalty free for any use with open source database systems such as MySQL, PostgreSQL, SQLite, etc. Licenses for using HTSQL in conjunction with proprietary databases such as Microsoft SQL and Oracle can be purchased.
DBeaver is a universal database manager and SQL Client. It supports MySQL, PostgreSQL, Oracle, DB2, MSSQL, Sybase, Mimer, HSQLDB, SQLite, Derby, and any database that has a JDBC driver. It is a GUI program that allows you to view the structure of a database, execute SQL queries and scripts, browse and export table data, handle BLOB/CLOB values, modify database meta objects, etc. It has a native UI (provided by the Eclipse SWT library), great performance, and relatively low memory consumption.
PDO4You is a class that implements the Singleton design pattern for connecting the database using the PDO extension (PHP Data Objects). It provides an abstraction layer for accessing data, regardless of which database you are using, and ensures that there is a single object instance per database connection. It can connect to several possible SQL databases using the available PDO drivers. Currently it supports MySQL, PostgreSQL, SQLite, MariaDB, Oracle, Microsoft SQL server, Sybase, and CUBRID (a database highly optimized for Web applications). The class can also execute multiple queries at once given a string in JSON format that specifies the type of query, the tables, fields, field values, conditions, etc.