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"

This syntax is specific to Informix SQL. Other database server types provide a function that extracts substrings from a character string.

Table 1. Substrings support by database server brand
Database Server Type	Substring function	col[x,y] support
IBM Informix	`SUBSTR(expr,start,length)`	Yes, native SQL feature
Microsoft™ SQL Server	`SUBSTRING(expr,start,length)`	Emulated, see details
Oracle® MySQL / MariadDB	`SUBSTR(expr,start,length)`	Emulated, see details
Oracle Database Server	`SUBSTR(expr,start,length)`	Emulated, see details
PostgreSQL	`SUBSTRING(expr FROM start FOR length) SUBSTR(expr,start,length)`	Emulated, see details
SAP HANA®	`SUBSTRING(expr,start,length)`	Emulated, see details
SQLite	`SUBSTR(expr,start,length)`	Emulated, see details
Dameng®	`SUBSTR(expr,start,length)`	Emulated, see details

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. Depending on the database types to be used, use SUBSTR() or SUBSTRING() as provided by the database engines.

You may also want to create your own user function with the databases engines that do not support the same syntax as most of your database engines, to have a common way to extract substrings. Note that with Microsoft SQL Server, a user function call must specify the owner as prefix. Therefore, when using SQL Server, instead of creating a SUBSTR() function, it is recommended that you create a SUBSTRING() function in other database types, since SQL Server provides SUBSTRING().