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Fill deleted numbers in consecutive series of numbers and LAST_INSERT_ID

First of all I'm an amateur and non-english native speaker, so I would appreciate it if you would have a little patience with me ;)
Trying to do two things here and I'm not sure if I should do two questions about it, but since it's all related in my case, I would like to say it all in one question.
I'm making a sort of accounting software, in theory for my personnal use. I'm using a DB generated auto_increment ID for almost all my objects, but for some specific cases I need a "parallel" more open ID that won't be primary key but could be manipulated by the user(yeah, I've read lots of questions about "you don't need a consecutive Primary Key", and i understand it and agree, but let me remark that this column won't be the primary key, lets call it just a "human-not-computer-expert friendly ID") matching these conditions:
The Id should auto increment when no parameters given.
When a number is given as a parameter that number should be used if not occupied, if occupied throw an exception.
The user should be asked if he/she wants to fill the missing IDs by DELETEs and whatever other operations, so if the user "say yes", the minimum missing ID should be automatically found and used.
I have no problem with doing this "by hand" in c#, but are there some way to achieve something like this in MySQL directly? I've read in the MySQL documentation that AUTO_INCREMENT does fulfill my first two conditions, but even if it fills missing deleted numbers by default, which I'm not sure of, I don't want it to do that by default, I need the software to ask first, or at least to do it based on a configuration pre established by the user.
Therefore I think I should do it by hand in c#(at least the last part, but i suspect i will be forced to do it entirely), which brings the question about LAST_INSERT_ID.
So, the MYSQL documentation says:
If the previous statement returned an error, the value of LAST_INSERT_ID() is undefined. For transactional tables, if the statement is rolled back due to an error, the value of LAST_INSERT_ID() is left undefined. For manual ROLLBACK, the value of LAST_INSERT_ID() is not restored to that before the transaction; it remains as it was at the point of the ROLLBACK.
I understand that LAST_INSERT_ID() is basically useless if the previous INSERT statement fails for whatever reason.
If that's the case, there's no way to retrieve the last inserted ID that ensures a known behaviour when something fails? Something like when INSERT fails returns 0 or a SQL exception? And if there's no other way what is the standard way of doing it(I suppose MAX(Id) won't do it), if something like a standard way exists... or should I just stop trying to do it at one go and do first the updates, check if all went ok, and then do a SELECT LAST_INSERT_ID?
To sum up:
Are there some way to achieve a column of consecutive numbers that fulfill the given conditions in MySQL directly?
What's with LAST_INSERT_ID? Should I give up and don't use it directly?

Situation 1, knowing an id that you want inserted into an AUTO_INCREMENT
Honoring that the AI is not a PK as described.
-- drop table a12b;
create table a12b
( id varchar(100) primary key,
ai_id int not null AUTO_INCREMENT,
thing varchar(100) not null,
key(ai_id)
);
insert a12b (id,thing) values ('a','fish'); -- ai_id=1
insert a12b (id,thing) values ('b','dog'); -- 2
insert a12b (id,thing) values ('b2','cat'); -- 3
delete from a12b where id='b';
insert a12b(id,ai_id,thing) values ('b',2,'dog with spots'); -- 2 ******** right here
insert a12b (id,thing) values ('z','goat'); -- 4
select * from a12b;
+----+-------+----------------+
| id | ai_id | thing |
+----+-------+----------------+
| a | 1 | fish |
| b | 2 | dog with spots |
| b2 | 3 | cat |
| z | 4 | goat |
+----+-------+----------------+
4 rows in set (0.00 sec)
Situation 2, having a system where you delete rows at some point. And want to fill those explicitly deleted gaps later: See my answer Here
Situation 3 (INNODB has a bunch of gaps sprinkled all over):
This was not part of the question. Perhaps use a left join utilizing a helper table (at least for ints not varchars. But then again we are talking about ints). If you need to spot a gap without knowing, shoot for a left join with a helper table (loaded up with numbers). I know it sounds lame, but helper tables are lean and mean and get the job done. The following would be a helper table: https://stackoverflow.com/a/33666394
INNODB Gap Anomaly
using the above table with 4 rows, continue with:
insert a12b (id,thing) values ('z','goat'); -- oops, problem, failed, but AI is incremented behind the scene
insert a12b (id,thing) values ('z2','goat'); -- 6 (you now have a gap)
data:
+----+-------+----------------+
| id | ai_id | thing |
+----+-------+----------------+
| a | 1 | fish |
| b | 2 | dog with spots |
| b2 | 3 | cat |
| z | 4 | goat |
| z2 | 6 | goat |
+----+-------+----------------+
There are a ton of ways to generate gaps. See This and That

How do I address an issue with an overly wide index?

I am not very proficient at SQL yet. I'm learning, but it's a slow process. I am working on a project at work which stores a good deal of information in a database in SQL Server. In one of the tables, ContactInformation, we're experiencing an error when an attempt to modify an entry runs afoul because a nonclustered index composed of all of the address information exceeds 900 bytes. I've used sys.dm_db_index_usage_stats to verify that modifying an entry in the table leads to 3 user_seeks and 1 user_update.
The C# code does not seem to be directly calling the index. It executes a single DbCommand that consists of a stored procedure command of the Update variety with 19 parameters. My thoughts are to either eliminate the index or to try to break up the DbCommand into multiple updates with a smaller number of parameters in hopes of having a smaller index to work with.
I am a bit at sea due to my lack of experience. I welcome any advice on which way to turn next.
The Index consists of the following:
| Name | Data Type | Size |
|----------------------|---------------|------|
| ContactInformationID | int | 4 |
| CompanyID | smallint | 2 |
| Address1 | nvarchar(420) | 840 |
| Address2 | nvarchar(420) | 840 |
| City | nvarchar(420) | 840 |
| State | nvarchar(220) | 440 |
| PostalCode | nvarchar(120) | 240 |
| Country | nvarchar(220) | 440 |
Yes, most of the columns are oversized. We apparently inherited this database from a different project. Our software limits most of the columns to no more than 100 characters, although there are some outliers.

The index size limit only applies to the key columns. It applies to all B-Tree bases storage modes (NCI and CI). This limit exists to ensure a certain degree on tree fanout in order to bound the tree height.
If you don't need to seek on columns such as Address1 and Address2 (considering that they might be null as well) make those columns included columns.
The index key should never be longer than the shortest key prefix that results in a unique index. Every column after that never helps compared to that column being included.

If ContactInformationID is unique, which I have a feeling it very well could be, then having any other fields in the index is pointless.
Such an index is useful only for queries where the value of ContactInformationID is present as a query parameter, and when it is, the rest of the fields are immaterial.

How do you update a Dataset made up of joined tables

I have 3 tables simplified to the below:
main_table
id | attribute1_id | attribute2_id | price
attribute1_table
id | attribute_name
attribute2_table
id | attribute_name
I've created a view in Sql Server that joins the tables together to give me the following output:
main_table
id | attribute1_id | attribute2_id | attribute1_name | attribute2_name | price
The problem I have is I want to be able to show the data in a DataGridView and allow the price to be editable. But I've created a "view" which I take it this is not the correct thing to use (i.e it's called a "view" which doesn't sound editable?)
I know I could create my own script to go through and update only the "main_table" but I think there must be a way to use DataGridView / Linked datasets with joined tables?
-

Best thing I can advise is to create a stored procedure that takes all of the parameters and then within that procedure do the individual update statements to the table. Should work fairly well.

How to make a blank database for every registered user in ASP MVC4?

How to make a blank database for every registered user or it is already implemented in ASP MVC4? Will they have different databases? For example, in an application, which stores contacts, I would like, that different users could create their own contact list.
And if I have ContactsController, which I want only registered users to be edited, should I write something like that?
[Authorize]
public class ContactsController : Controller
EDIT
O thanks Bhushan, I didn't know that User ID will store different data.In Contact table I have ContactId, which is associated with a contact data and it has a primary key, should I add for example UserId in the same Contact table to associate it with registered user ID? And should UserId have primary key?

If I understand your question properly:
different users could create their own contact list
Above task doesn't require a separate database, You can associate Contacts with Users by adding a field ContactOwner or similar name in the Contact table which will store a User ID.
So your contacts table can look like following: (TABLE NAME: CONTACTS)
|ContactID | ContactType | ContactName | ContactOwner |
|0123456789 | MOBILE | Mr. ABC | USER1 |
|email#email.com | EMAIL | Mr. ABC | USER1 |
|0123456789 | MOBILE | Mr. PQR | USER1 |
|0123456789 | MOBILE | Mr. XYZ | USER1 |
|0123456789 | MOBILE | Mr. LMN | USER2 |
|0123456789 | MOBILE | Mr. AAA | USER3 |
So as you can see the sample table above you can identify which ContactID belongs to which User? Or which are the contacts created by which User?
In this case you have to make ContactOwner primary key(Composite key) with other primary key(s), to make sure that one contact can be Owned/Created by multiple Users.
Update 1
To get contacts created by specific user you can write a query like:
Select * from CONTACTS where ContactOwner = 'USER1'
Above query will give you the contacts which are Created/Owned by USER1, and not the ones created by other users. so your result will contain the following records using above query:
|ContactID | ContactType | ContactName | ContactOwner |
|0123456789 | MOBILE | Mr. ABC | USER1 |
|email#email.com | EMAIL | Mr. ABC | USER1 |
|0123456789 | MOBILE | Mr. PQR | USER1 |
|0123456789 | MOBILE | Mr. XYZ | USER1 |
Update 2
Your query will be a dynamic one in which ContactOwner will be the USER who is logged in. This and This might help you to write dynamic query (prepared statement)[Note: since I am a Java developer so I don't know the suitable syntax in C# for writing the dynamic query.]

If you want to have an empty DB for each user, just add as part of the registration process creating new database. you can easily do that with SQL statements (CRATE DATABASE, followed by CREATE TABLE and other create statements to create your schema). you can also use SMO do duplicate existing template databse. you can also create it using entity framework database-first (with few tweaks, like modifying your connection string). BUT, all those options are not recommended for your scenario.
If you want to give each user his contact list, just add to the "Contacts" table "OwnerUserID" column, or something like that, that will have FK relationship to your users table, and will include the user which this row belongs to. In your queries, just filter by this and show each user his own contact list.
This way it'll be much easier for you to manage things and add/modify things in the future. also, working with multiple databases like you asked in your questions will give you a lot of pain, with almost no benefits (in most scenarios).
Best of luck.

FYI, in e.g. SQL Server 2012, the maximum number of databases is 32,767.
Please don't make a new database for every user, it defies basic logic and the point of having relational databases in the first place. Use tables and foreign keys and whatnot.

What is the best way to add/update/delete a lookup table in Microsoft SQL with ASP.NET C#?

I'm working on a local city project and have some questions on efficiently creating relationships between "parks" and "activities" in Microsoft SQL 2000. We are using ASP.NET C# to
I have my two tables "Parks" and "Activities." I have also created a lookup table with the proper relationships set on the primary keys of both "Parks" and "Activities." My lookup table is called "ParksActitivies."
We have about 30 activities that we can associate with each park. An intern is going to be managing the website, and the activities will be evaluated every 6 months.
So far I have created an admin tool that allows you to add/edit/delete each park. Adding a park is simple. The data is new, so I simply allow them to edit the park details, and associate "Activities" dynamically pulled from the database. This was done in a repeater control.
Editing works, but I don't feel that its as efficient as it could be. Saving the main park details is no problem, as I simply call Save() on the park instance that I created. However, to remove the stale records in the lookup table I simply DELETE FROM ParksActitivies WHERE ParkID = #ParkID" and then INSERT a record for each of the checked activities.
For my ID column on the lookup table, I have an incrementing integer value, which after quite a bit of testing has got into the thousands. While this does work, I feel that there has to be a better way to update the lookup table.
Can anyone offer some insight on how I may improve this? I am currently using stored procedures, but I'm not the best at very complex statements.
[ParkID | ParkName | Latitude | Longitude ]
1 | Freemont | -116.34 | 35.32
2 | Jackson | -116.78 | 34.2
[ActivityID | ActivityName | Description ]
1 | Picnic | Blah
2 | Dancing | Blah
3 | Water Polo | Blah
[ID | ParkID | ActivityID ]
1 | 1 | 2
2 | 2 | 1
3 | 2 | 2
4 | 2 | 3
I would prefer to learn how to do it a more universal way as opposed to using Linq-To-SQL or ADO.NET.

would prefer to learn how to do it a more universal way as opposed to using LINQ2SQL or ADO.NET.
You're obviously using ADO.NET Core :). And that's fine I think you should stick to using Stored procedures and DbCommands and such...
If you were using MSSQL 2008 you'd be able to do this using TableValued parameters and the MERGE statement. since you're using MSSQL 200 (why?) what you'd need to do is the following:
1. Send a comma delimited list of the Activity ids (the new ones) along with the ParkId to your stored proc. The ActivityIds parameter would be a varchar(50) for example.
In your stored proc you can split the ids
The strategy would be something like
1. For the Ids passed in, delete records that don't match
The SQL for that would be
DELETE FROM ParkActivities
WHERE ActivityId NOT IN (Some List of Ids)
WHERE ParkId = #ParkId
Since your list is a string you can do it like this
EXEC('DELETE FROM ParkActivities WHERE ActivityId NOT IN (' + #ActivityIds + ') AND ParkId = ' + #ParkId)
Now you can insert those activities that are not already in the table. The simplest way to do this would be to insert the ParkActivity ids into a temp table. To do that you'll need to split the comma delimited list into individual ids and insert them into a temp table. Once you have the data in the temp table you can insert doing a join.
The is a built-in user defined function in MSSQL 2000 that can do the split and return a Table Variable with each value on a seperate row.
http://msdn.microsoft.com/en-us/library/Aa496058

What is wrong with LinqToSQL and ADO.NET? I mean, could you specify your doubts about using those technologies
update
if LinqToSQL is not supported for 2000, you can easily upgrade to free 2008 express. It would be definitely enough for purposes you described.