Substring expressions

Handle substrings expressions with different database engines.

Only IBM^® Informix^® supports substring specification with square brackets:

SELECT * FROM item WHERE item_code[1,4] = "XBFG"

However, most database servers support a function that extracts substrings from a character string:

Table 1. Database server support of extraction of substrings
Database Server Type	Supports col[x,y] substrings?	Provides substring function?
Genero db	Yes	SUBSTR(expr,start,length)
IBM DB2^® UDB	No	SUBSTR(expr,start,length)
IBM Informix (1)	Yes	SUBSTR(expr,start,length)
Microsoft™ SQL Server (2)	No	SUBSTRING(expr,start,length)
MySQL	No	SUBSTR(expr,start,length)
Oracle Database Server	No	SUBSTRING(expr,start,length)
PostgreSQL	No	SUBSTRING(expr FROM start FOR length )
Sybase ASE	No	SUBSTRING(expr,start,length)
SQLite	No	SUBSTR(expr,start,length)

Informix allows you to update some parts of a CHAR and VARCHAR column by using the substring specification ( UPDATE tab SET col[1,2] ='ab' ). This is not possible with other databases.

Review the SQL statements using substring expressions and use the database specific substring function.

You could also create your own SUBSTRING() user function in all databases that do not support this function, to have a common way to extract substrings. In Microsoft SQL Server, when you create a user function, you must specify the owner as prefix when using the function. Therefore, you should create a SUBSTRING() user function instead of SUBSTR().