Is there any way to use T-SQL queries with MySQL database, like having a data connector that understand TSQL and can connect to MySQL?
Short answer, no.
By the time you've developed or found an interface that could translate T-SQL syntax into MySQL (correctly); you might as well have learnt the syntax required to write the MySQL you need.
It's not all that different to be honest; and it will broaden your knowledge and make you flexible for other types of database query languages.
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I'm not very versed on databases, so thismight sound wrong to some of you: Can I use SQL, MySQL and/or SQLite to read the same database? If so, are there commands or instructions I should keep an eye on to not make a mess on the tables?
Thanks in advance!
sql is a language. sqlite and mysql are database engines.
Both SQLite and MySQL (as far as any SQL engines) allows SQL language to manipulate database content (with some engine specific ).
So you may use SQL to read a MySQL or SQLite database. But be aware that SQL use in each is engine dependent. For instance, in SQLite you may use shell application, c wrapper, ... For MySQL you may use php wrapper, ...
I am thinking to write a converter that takes any oracle query and return Postgresql format of the query assuming table and columns are same.
what I do right now I do timely conversions so I have basic understanding about both and want some expert advice that is it easily possible or not?
Try to use "commercial" version of PostgreSQL - EnterpriseDB. It has an compatibility layer for Oracle.
If you're about to write the "convector" by your own: look at this github project: https://github.com/porcelli/plsql-parser. It's open-source parser for Oracle's SQL dialect. I have to warn you, even if you have AST for Oracle query it is still a lot of to do to convert AST into other SQL dialect. You will also need plenty of sample queries for testing. You can find some sample queries in this project's tests folder.
Also similar project was implemented for MySQL, but I can not find it's homepage now.
Part of the solution is to make available in PostgreSQL the functions available in Oracle. You can have a look at http://orafce.projects.pgfoundry.org/
"The goal of this project is to implemente some functions from Oracle database. Some date functions (next_day, last_day, trunc, round, ...), string functions and some modules (DBMS_ALERT, DBMS_OUTPUT, UTL_FILE, DBMS_PIPE, ...) are implemented now. Funcionality was verified on Oracle 10g and module is useful for production work."
Not possible for every query. They each have syntax and functionality that the other does not -- for example, the MODEL clause in Oracle, or PostgreSQL's special form of "SELECT DISTINCT ON".
Mostly, Oracle has functionality that PostgreSQL doesn't: http://docs.oracle.com/cd/B28359_01/server.111/b28286/statements_10002.htm
I've seen few posts here saying I can do hierarchical queries using mysql.
I've not used PostgreSQL, yet. If I change to PostgreSQL, do I have to change a lot of syntax or can I do that hierarchical stuff in MySQL itself?
Hierarchical queries in MySQL
What sender sends is Facebook invite so can I do this in SQL?
MySQL doesn't have recursive queries, databases like PostgreSQL, SQL Server and Oracle do have these types of queries. Very powerful and easy to use, but in MySQL you're on your own.
You could use a work around like a nested set, works in almost any database but is not as fast.
Are there any SQL Validators that can check syntax against multiple database servers?
For example, I might want to check whether a particular query will work against Oracle, MySQL and SQL Server.
I am more concerned about the SQL syntax than the actual schema being queried, so a tool that can catch major syntax errors, like detecting that the limit clause is not supported in SQL Server and Oracle would be good enough.
EDIT:
Several answers have suggested checking syntax for a particular DBMS or for ANSI Standard.
What I am looking for a possibility of targeting a sort of union of features between two or more databases (say SQL Server and Oracle). I would like to be able to use any SQL feature that is supported by all of the DBMS I am targeting in a particular application.
I am not sure if it is worthwhile. I think it depends upon the number of non-ANSI features shared by several databases. If there are very few, then perhaps it will be better to target the ANSI standard.
I'm not aware of any that are that specific, these will check that the statements are valid ansi 92/99/2003...
http://developer.mimer.com/validator/index.htm
That will get you 99% of the way there (especially if are only doing CRUD operations)
maybe if you know which reserved words are used by which database you could roll your own simple checker..
see : How to find if a column name is a reserved keyword across various databases
(as already mentioned) If your goal is to create a database agnostic system think about using a third party tool e.g: entityspaces
For the MySQL: try automatic SQL syntax check feature in dbForge Studio for MySQL.
For the Oracle: PL/SQL Editor in dbForge Studio for Oracle
This is an online mysql syntax checker, but this is mysql only
http://www.piliapp.com/mysql-syntax-check/
You can use https://www.db-fiddle.com/. It supports the following DBs
MySQL 8.0
MySQL 5.7
MySQL 5.6
MySQL 5.5
PostgreSQL 13
PostgreSQL 12
PostgreSQL 11
PostgreSQL 10.0
PostgreSQL 9.6
PostgreSQL 9.5
PostgreSQL 9.4
SQLite 3.30
SQLite 3.26
I am working in a project where we are using SQL Server database currently. But recently a decision has been taken that the database will be changed to MySQL.
I am not using any stored procedures, views, triggers, user defined functions, etc. But I think even then some queries written for SQL Server will not be supported by MySQL.
Can anyone help: what are the things that I have to check (and change) so that all the queries will work properly for MySQL also?
Queries that I know without consulting the documentation that will not work:
(recursive) common table expressions
windowing functions
queries using the standard SQL string concatenation ||
UPDATEs with JOIN are different between the two systems
Date arithmetics: date_column + 1 behaves differently in SQL Server
Division by zero will produce an error
SQL Server will reject values that do not fit into a column (instead of silently truncating it, which MySQL does in the default installation)
DDL that will not work and might have an impact on performance and/or data quality
datetime columns where you need precision up to milliseconds
tables with check constraints
indexed views
triggers on views
table functions (select * from my_function(42);)
filtered indexes ("partial index")
function based indexes
There's always the option to take commercial support from MySQL AB for this problem. I'm pretty sure they've done enough MSSQL->MySQL migrations to know alot about that. If a price tag on the migration is not a problem.
Alternatively, you could try to run the MySQL Migration Toolkit over the data and look for meaningful error messages at the stuff it cannot migrate. MySQL Migration Toolkit is part of the MySQL GUI Tools.