Admittedly, this documentation is quite incomplete. But since this interface is standardized, additional information can be found in many resources about SQL.
An embedded SQL program consists of code written in an ordinary programming language, in this case C, mixed with SQL commands in specially marked sections. To build the program, the source code is first passed to the embedded SQL preprocessor, which converts it to an ordinary C program, and afterwards it can be processed by a C compilation tool chain.
Embedded SQL has advantages over other methods for handling SQL commands from C code. First, it takes care of the tedious passing of information to and from variables in your C program. Second, the SQL code in the program is checked at build time for syntactical correctness. Third, embedded SQL in C is specified in the SQL standard and supported by many other SQL database systems. The PostgreSQL implementation is designed to match this standard as much as possible, and it is usually possible to port embedded SQL programs written for other SQL databases to PostgreSQL with relative ease.
As indicated, programs written for the embedded SQL interface are normal C programs with special code inserted to perform database-related actions. This special code always has the form
EXEC SQL ...;
These statements syntactically take the place of a C statement. Depending on the particular statement, they may appear in the global context or within a function. Embedded SQL statements follow the case-sensitivity rules of normal SQL code, and not those of C.
The following sections explain all the embedded SQL statements.