I want to get the first eight characters of a text field, but I also need to get more values for other fields of...
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three tables. I don't know what form of the "substring" function is correct to use. Or is there another function that I could use?
The standard SQL substring function is
SUBSTRING(string FROM start FOR length)
Microsoft SQL/Server, Sybase, MySQL and PostgreSQL all support this syntax. Other databases may also support it (I have not done an exhaustive survey). Double-check by looking it up in your database documentation.
In Microsoft SQL/Server, Sybase, and MySQL, you can also use
SUBSTRING(string, start, length)
In Microsoft Access and MySQL, you can use
MID(string, start, length)
In Oracle, use
SUBSTR(string, start, length)
A good reference for SQL functions is Chapter 4, SQL Functions, the sample chapter of O'Reilly's book SQL in a Nutshell by Kevin Kline and Daniel Kline (ISBN 1-56592-744-3).
For your specific requirement, you would write
SUBSTRING(string FROM 1 FOR 8)
Depending on your database, you might also be able to write
because the LEFT function, in those databases that support it, starts at position 1.
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