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PODynamically Selecting value from column table based on column name from other table (reflection)
 

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  1. PODynamically Selecting value from column table based on column name from other table (reflection)
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    <p>Assume I have 2 tables FOO and BAR as below, is it possible to use a sort of reflection on FOO, if you know the column name as a string based on a join of the BAR table?</p> <pre><code>SELECT DB, FOO.Name, FOO.Type, BAR.Field, I.DATA_TYPE AS FType, FOO.**&lt;BAR.FIELD&gt;** AS Value FROM INFORMATION_SCHEMA.COLUMNS AS I inner JOIN BAR ON I.COLUMN_NAME = BAR.Field inner JOIN FOO ON FOO.TYPE = BAR.TYPE WHERE DB = 4 AND FLAG = 1 </code></pre> <p>i.e. for each selected row, FOO.<strong></strong> need to change to reflect the value of the matching column in FOO, i.e. if one row has BAR {4, AC1, LO} and FOO { 4, AC1, LO, COL1} I want the value of 1 to be picked.</p> <p>I know that I can probably do this is 2 rounds and merge the the data, however I wondered if anybody would know of a way to do this more efficiently in 1 go, saving code path.</p> <p>I should add, I generally have around 60 columns in either table, and they are pretty random, i.e. I cannot assume that either col1, 2 or 3 exist, I can only go by what is in the equivalent BAR table.</p> <p>FOO:</p> <pre><code>+--------+--------+---------+---------+--------+-------+ | DB | Name | Type | Col1 | Col2 | Col3 | +--------+--------+---------+---------+--------+-------+ | 4 | AC1 | LO | 1 | 10 | 2 | | 4 | AC1 | HI | 2 | 20 | 4 | | 1 | DC2 | HI-HI | 11 | 5 | 2 | | 1 | DC2 | HI | 22 | 10 | 4 | | 1 | DC2 | LO | 33 | 15 | 6 | +--------+--------+---------+---------+--------+-------+ </code></pre> <p>BAR:</p> <pre><code>+--------+--------+---------+---------+--------+ | DB | Name | Type | Field | Flag | +--------+--------+---------+---------+--------+ | 4 | AC1 | LO | Col1 | 1 | | 4 | AC1 | HI | Col1 | 1 | | 1 | DC2 | HI-HI | Col1 | 1 | | 1 | DC2 | HI | Col1 | 1 | | 1 | DC2 | LO | Col1 | 1 | | 4 | AC1 | LO | Col2 | 0 | | 4 | AC1 | HI | Col2 | 0 | | 1 | DC2 | LO | Col2 | 0 | | 1 | DC2 | HI-HI | Col2 | 0 | | 1 | DC2 | HI | Col2 | 0 | | 4 | AC1 | LO | Col3 | 0 | | 4 | AC1 | HI | Col3 | 0 | | 1 | DC2 | LO | Col3 | 0 | | 1 | DC2 | HI-HI | Col3 | 0 | | 1 | DC2 | HI | Col3 | 0 | +--------+--------+---------+---------+--------+ </code></pre> <p>RESULT:</p> <pre><code>+--------+--------+---------+---------+--------+--------+ | DB | Name | Type | Field | FTYPE | VALUE | +--------+--------+---------+---------+--------+--------+ | 4 | AC1 | LO | Col1 | float | 1 | | 4 | AC1 | HI | Col1 | float | 2 | | 4 | AC1 | LO | Col2 | float | 10 | | 4 | AC1 | HI | Col2 | float | 20 | | 4 | AC1 | LO | Col3 | float | 2 | | 4 | AC1 | HI | Col3 | float | 4 | +--------+--------+---------+---------+--------+--------+ </code></pre>
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    1. USaggaton
    1. USaggaton
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    1. CO"is it possible to use a sort of reflection on FOO, if you know the column name as a string based on a join of the BAR table?" `Yes, but it requires the use of `[dynamic SQL](http://stackoverflow.com/questions/4165020/what-is-dynamic-sql)
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      1. USxQbert
    2. COSo you are suggesting building the SQL on the DAL side, inserting a select block per column name and then stitch all the selects together with UNION ALL's? That is sort of what I was trying to avoid, since it would still require 2 calls, for me to get a list of all the possible column names.
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      1. USaggaton
    3. COthere is no way besides dynamic sql for that thing on server. You can create sp with that dynamic stuff and use it from client.
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      1. USshibormot
 

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