SQL

General

What is more heavy for the database? Reads or writes? What needs to be considered to answer such question?

What is a SQL injection? How to prevent?
Types of joins (left, right, inner, full)
What is the difference between LEFT INNER JOIN and LEFT OUTER JOIN?
What is union? Union distinct
What is index in database?
What data structures a db index can be based on?
What is "having" in SQL? When to use it?
Where to use "having" and when "where"
How can you display SQL query results so that they are not repeated (distinct)?
What are views? How to create? Is it possible to update,insert?
What are: candidate key, primary key, composite key, foreign key, surrogate key?
ACID
What is a bulk insert?
Describe aggregate functions
What is a pagination of records?
What is a sequence?
READ_COMMITED vs READ_COMMITED_SNAPSHOT in MS SQL Server

What types of NoSQL databases do you know?
When to use NoSQL database comparing to relational one?
How could you compare the schema definition in NoSQL comparing to relational databases?
What is the CAP theorem in distributed system?

What is syntax tree?
What is execution plan? How can we preview the execution plan?
What algorithms are used to calculate execution plan?
Describe master-slave replication.
Describe multi-master replication.
Describe what sharding is.
How would you improve performance of db that is only used to store data and not retrieving.
Describe how SQL statement is processed by database (Parser -> Optimizer -> Executor)
How is SQL statement optimized?
Are execution plans cached in databases?
What data structures are used in indexes? (trees, hashes etc.)

How can you map objects with inheritance to tables? What are the pros/cons of each approach? (Compare approaches from hibernate inheritance strategies). What is a discriminator column?

What algorithms do you know that allow to evenly distribute load between shards? (i.e. hash-based)
How can we add another shard and evenly distribute data over there? (without shutting down entire cluster)