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Programming Interfaces
Data Management : Database Access : Joining tables : Specifying joins in ABL : Using joins instead of nested reads
Using joins instead of nested reads
Using joins provides an opportunity for the AVM to optimize the retrieval of records from multiple related tables using the selection criteria you specify. When you perform a nested read, for example using nested FOR EACH statements for different tables, you are actually implementing a join in an ABL procedure. However, by specifying one or more contiguous joins in a single FOR EACH statement or in the PRESELECT phrase of single DO or REPEAT statement, you minimize the complexity of your ABL code and leave the complexity of joining tables to the ABL interpreter.
For a single ABL query (OPEN QUERY statement), there is no other way to retrieve data from multiple tables except by using joins. With both inner and left outer join capability, you can use the OPEN QUERY statement to implement most queries that are possible using nested FOR EACH, DO, or REPEAT statements. As such, query joins provide the greatest opportunity for optimized multi-table record retrieval in ABL. Also, because browse widgets read their data from queries, you must use query joins to display multiple related tables in a browse.
However, use nested FOR EACH, DO, and REPEAT blocks wherever you require much finer control over how you access and manipulate records from multiple tables.