PRIMARY acts exactly like a UNIQUE index, except that it is always named 'PRIMARY', and there may be only one on a table (and there should always be one though some database systems don't enforce this). Depending on your application, however, you may find this undesirable: if you wish to prevent this, you should disallow NULL values in the relevant columns. Your database system may allow a UNIQUE index to be applied to columns which allow NULL values, in which case two rows are allowed to be identical if they both contain a NULL value (the rationale here is that NULL is considered not equal to itself). As well as being used to quickly reach certain record ranges, UNIQUE indexes can be used to enforce restraints on data, because the database system does not allow the distinct values rule to be broken when inserting or updating data.
That is, the same row may not have identical non-NULL values for all columns in this index as another row. UNIQUE refers to an index where all rows of the index must be unique. These indexes don't enforce any restraints on your data so they are used only for access - for quickly reaching certain ranges of records without scanning all records. Non-distinct values for the index are allowed, so the index may contain rows with identical values in all columns of the index. KEY or INDEX refers to a normal non-unique index. I've been able to generate strings by defining a string containing all the allowed chars and randomly substringing it, and nothing more. I thought of the generate&check loop approach again, but I'm not limiting this question to that just in case there's a more efficient one. The query I need should simply generate a 8-character alphanumeric string which is not already in the table. Therefore, I decided to try and solve this issue using a MySQL query. However, as the number of vehicles increases, I expect this to become even more inefficient it is right now. How I achieved this was using a while loop in Lua, which is the language I'm programming in, to generate strings and query the DB to see if it is used.
Mysql generate random string license#
I need to find an unused license plate before creating a new vehicle – it should be an alphanumeric 8-char random string.
Now here comes the part I'm having problems with. I have a MySQL table named "vehicles" containing the data about the vehicles, including the column "plate" which stores the License Plates for the vehicles. I'm working on a game which involves vehicles at some point.
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