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What means this query?
@numberx = @numberx -1
UPDATE th
SET @numberX= numberY= @numberX + 1
FROM Table1 th
INNER JOIN Table2 td ON th.Id = td.idth
WHERE td.anything = @anything
At line 3, what is this "double equality"? And what is this "from" and "inner" on an UPDATE?
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The EXPLAIN keyword is used throughout various SQL databases and provides information about how your SQL database executes a query. In MySQL, EXPLAIN can be used in front of a query beginning with SELECT , INSERT , DELETE , REPLACE , and UPDATE .
The @CustID means it's a parameter that you will supply a value for later in your code. This is the best way of protecting against SQL injection. Create your query using parameters, rather than concatenating strings and variables.
A query can either be a request for data results from your database or for action on the data, or for both. A query can give you an answer to a simple question, perform calculations, combine data from different tables, add, change, or delete data from a database.
It's a multiple table update for assigning consecutive numbers to each row in Table1 where the corresponding row in Table2 has a specific value for the column anything.
For each matching row it sets the column numberY to the value of @numberX + 1. It also reassigns that value back to @numberX, which causes @numberX to be incremented for each row.
This is known as a "quirky update". It is an undocumented and not guaranteed approach to generate running totals. In 2012 SUM() OVER (ORDER BY ...) should be used instead.
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