For Each commandOfficial Content

The GeneXus procedural language command used to get a dataset from the database. When used in procedures, besides reading the database, it can also be used in order to update it.

Within a For Each command we define the information we want to read (also write if inside a Procedure object) by specifying the Transaction Level from which we want to retrieve information, and the name of the attributes we want to use. In addition, it has many optional clauses that lets you fine tune your query. 

Syntax

For each [<BaseTrnList>]

[SKIP <Expression> COUNT <Expression>]

[<OrderList>]

[<UsingDataSelector>]

[<UniqueAttributeList>]

[<ConditionList>]

[<BlockingGroup>]

<MainCode>

[When duplicate

[<CodeWhenDuplicate>]]

[When none

[<CodeWhenNone>]]

Endfor

 

Where:

<BaseTrnList> ::=
 <Trn1> {, <Trni>}...

i.e. a list of Transaction Level names. See more on Base Transaction clause.

 

<OrderList> ::=

{ {order <attributeList>i [when <cond>i] } | [order none] [when <cond>x] }...

Allows to indicate the order in which the query will be returned. As the syntax indicates, you can write many conditional order clauses as well as undefined order (order none).

<UsingDataSelector> ::=
using <DataSelectorName>( [ <parameterList> ] )

Allows ordering & filtering according to the criteria set out in the Data selector specified by its name <DataSelectorName>.

<UniqueAttributeList> ::=
 unique <attributeList>

Returns only those records where the set of values of the referred attributes is unique. See more on Unique Clause.

<ConditionList> ::=
{where {<condition> [when <condition>] } |
  { [not]<attribute> in <DataSelectorName>( [<parameterList>]) }
}...  

Allows to indicate a list of conditions that must be matched by the data in order to be processed within the for each (inside the <MainCode>). The conditions are boolean expressions, simple ones or very complex nested ones. 
See more on Where clause (<condition> syntax included).

<BlockingGroup> ::=
blocking <numericExpression>

Available in procedures (when updating/deleting). It indicates the amount of records that will constitute a database block, when you are updating or deleting, in order to reduce the number of roundtrips to the DBMS. See more on Blocking clause.

<MainCode> ::=
<commandList>
 
<CodeWhenDuplicate> ::= 
<commandList>

Available in procedures, when inside <MainCode> certain attribute is updated (overwritten) and there exists a unique index for it. When you intend to overwrite the record with a previously existing value, this code will be executed instead of <MainCode> for that record. See more on When duplicate clause

<CodeWhenNone> ::=
<commandList>

When there is no data meeting the filter conditions, the <MainCode> commands will not be executed; the <CodeWhenNone> commands will be executed instead. See more on When None Clause.

Note

The Defined By Clause from previous versions of GeneXus (located between <ConditionList> and <BlockingGroup> in syntax), although allowed, becomes meaningless against the Base Transaction clause.

Optimizations

Boolean expressions in the where clause constrain the dataset that is obtained. However, more than that is meant here. There is a great difference between a full scan table access and an index-optimized random access. If applying some rules of logic, GeneXus determines that the order clause (or the primary key order) is compatible with the where boolean expression(s), it optimizes the access to the table using indices and not doing a full table scan. This is obviously the desired behavior whenever possible.

See Conditional Orders and Filters for further information.

Smart Devices applications

For Each command can be used when developing applications for Smart Devices Generator but with some considerations. It can only be used in Work With for Smart Devices object and Panel for Smart Devices object and only in the System Smart Device Events, this means Start, Refresh and Load events.

See Also

Nested For Each commands

Videos

Start Video Introduction to procedures, lists and For Each command
Start Video Simple For Each command and nested For Each command to list related info
Start Video Data Base Update - For Each, Delete, New

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