SQL constraints are important for data validation. Ensuring the validity of the data in your database means doing more than just making sure the data is of the right type. Perhaps some columns, for example, should never hold a null value — and maybe others should hold only values that fall within a certain range.

SQL joins are extremely useful. Unlike the other kinds of SQL join, the union join makes no attempt to match a row from the left source table with any rows in the right source table. It creates a new virtual table that contains the SQL union of all the columns in both source tables. In the virtual result table, the columns that came from the left source table contain all the rows that were in the left source table.

The first thing to understand about SQL is that SQL isn’t a procedural language, as are Python, C, C++, C#, and Java. To solve a problem in a procedural language, you write a procedure — a sequence of commands that performs one specific operation after another until the task is complete. The procedure may be a straightforward linear sequence or may loop back on itself, but in either case, the programmer specifies the order of execution.

To use a SQL cursor, you first must declare its existence to the DBMS. You do this with a DECLARE CURSOR statement. The DECLARE CURSOR statement doesn't actually cause anything to happen; it just announces the SQL cursor’s name to the DBMS and specifies what query the cursor will operate on. A DECLARE CURSOR statement has the following syntax: DECLARE cursor-name [] [] CURSOR [] [] FOR query expression [ORDER BY order-by expression] [FOR updatability expression] ; The cursor name uniquely identifies a cursor, so it must be unlike that of any other cursor name in the current module or compilation unit.

ODBC is a standard interface between a SQL database and an application that accesses the data in the database. Having a standard enables any application front end to access any database back end by using SQL. The only requirement is that the front end and the back end both adhere to the ODBC standard. ODBC 4.0 is the current version of the standard.

SQL is a popular and useful programming language. You can make SQL even more useful if you know the phases of SQL development, the criteria for normal forms, the data types used by SQL, a little bit about set and value functions, as well as some tips on how to filter tables with WHERE clauses.Phases of SQL system developmentIn developing any system, you start at the beginning and go through to the end, and it’s no different with SQL.