DYnamic SQL examples

I have lately learned what is dynamic sql and one of the most interesting features of it to me is that we can use dynamic columns names and tables. But I cannot think about useful real life examples. The only one that came into my mind is statistical table.

Let`s say that we have table 开发者_运维百科with name, type and created_data. Then we want to have a table that in columns are years from created_data column and in row type and number of names created in years. (sorry for my English)

What can be other useful real life examples of using dynamic sql with column and table as parameters? How do you use it?

Thanks for any suggestions and help :) regards Gabe

/edit Thx for replies, I am particulary interested in examples that do not contain administrative things or database convertion or something like that, I am looking for examples where the code in example java is more complicated than using a dynamic sql in for example stored procedure.

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An example of dynamic SQL is to fix a broken schema and make it more usable.

For example if you have hundreds of users and someone originally decided to create a new table for each user, you might want to redesign the database to have only one table. Then you'd need to migrate all the existing data to this new system.

You can query the information schema for table names with a certain naming pattern or containing certain columns then use dynamic SQL to select all the data from each of those tables then put it into a single table.

INSERT INTO users (name, col1, col2)
SELECT 'foo', col1, col2 FROM user_foo
UNION ALL
SELECT 'bar', col1, col2 FROM user_bar
UNION ALL
...

Then hopefully after doing this once you will never need to touch dynamic SQL again.

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Long-long ago I have worked with appliaction where users uses their own tables in common database. Imagine, each user can create their own table in database from UI. To get the access to data from these tables, developer needs to use the dynamic SQL.

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I once had to write an Excel import where the excel sheet was not like a csv file but layed out like a matrix. So I had to deal with a unknown number of columns for 3 temporary tables (columns, rows, "infield"). The rows were also a short form of tree. Sounds weird, but was a fun to do. In SQL Server there was no chance to handle this without dynamic SQL.

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Another example from a situation I recently came up against. A MySQL database of about 250 tables, all in MyISAM engine and no database design schema, chart or other explanation at all - well, except the not so helpful table and column names.

To plan for conversion to InnoDB and find possible foreign keys, we either had to manually check all queries (and the conditions used in JOIN and WHERE clauses) created from the web frontend code or make a script that uses dynamic SQL and checks all combinations of columns with compatible datatype and compares the data stored in those columns combinations (and then manually accept or reject these possibilities).