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transpose a table using SQL

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Cards (15)

Section 1

(15 cards)

transpose a table using SQL

Front

use CASE statement or DECODE statement.

Back

Self Join and why is it required

Front

Self Join is the act of joining one table with itself. Self Join is often very useful to convert a hierarchical structure into a flat structure In our employee table example above, we have kept the manager ID of each employee in the same row as that of the employee. This is an example of how a hierarchy (in this case employee-manager hierarchy) is stored in the RDBMS table. Now, suppose if we need to print out the names of the manager of each employee right beside the employee, we can use self join.

Back

foreign key

Front

one table which can be related to the primary key of another table. Relationship needs to be created between two tables by referencing foreign key with the primary key of another table.

Back

WHERE clause and HAVING clause?

Front

WHERE and HAVING both filters out records based on one or more conditions. The difference is, WHERE clause can only be applied on a static non-aggregated column whereas we will need to use HAVING for aggregated columns. Suppose we want to see only those departments where department ID is greater than 3. There is no aggregation operation and the condition needs to be applied on a static field. We will use WHERE clause here. Next, suppose we want to see only those Departments where Average salary is greater than 80. Here the condition is associated with a non-static aggregated information which is "average of salary". We will need to use HAVING clause here.

Back

Group by vs Partition by

Front

Group by- aggregate relationship of rows Partition by - works with relationship within columns

Back

unique key

Front

uniquely identified each record in the database. This provides uniqueness for the column or set of columns. can be many unique constraint defined per table, but only one Primary key constraint defined per table

Back

primary key

Front

combination of fields which uniquely specify a row. This is a special kind of unique key, and it has implicit NOT NULL constraint. It means, Primary key values cannot be NULL.

Back

difference between JOIN and UNION

Front

SQL JOIN allows us to "lookup" records on other table based on the given conditions between two tables. For example, if we have the department ID of each employee, then we can use this department ID of the employee table to join with the department ID of department table to lookup department names. UNION operation allows us to add 2 similar data sets to create resulting data set that contains all the data from the source data sets. Union does not require any condition for joining. For example, if you have 2 employee tables with same structure, you can UNION them to create one result set that will contain all the employees from both of the tables.

Back

UNION, MINUS and INTERSECT

Front

UNION combines the results from 2 tables and eliminates duplicate records from the result set. MINUS operator when used between 2 tables, gives us all the rows from the first table except the rows which are present in the second table. INTERSECT operator returns us only the matching or common rows between 2 result sets.

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Denormalization

Front

technique used to access the data from higher to lower normal forms of database. It is also process of introducing redundancy into a table by incorporating data from the related tables.

Back

difference between inner and outer join

Front

Inner join is the most common type of Join which is used to combine the rows from two tables and create a result set containing only such records that are present in both the tables based on the joining condition (predicate). If none of the record matches between two tables, then INNER JOIN will return a NULL set. Below is an example of INNER JOIN and the resulting set. Outer Join, on the other hand, will return matching rows from both tables as well as any unmatched rows from one or both the tables (based on whether it is single outer or full outer join respectively). Notice in our record set that there is no employee in the department 5 (Logistics). Because of this if we perform inner join, then Department 5 does not appear in the above result. However in the below query we perform an outer join (dept left outer join emp), and we can see this department.

Back

normalization

Front

process of minimizing redundancy and dependency by organizing fields and table of a database. The main aim of Normalization is to add, delete or modify field that can be made in a single table.

Back

UNION and UNION ALL

Front

UNION and UNION ALL both unify for add two structurally similar data sets, but UNION operation returns only the unique records from the resulting data set whereas UNION ALL will return all the rows, even if one or more rows are duplicated to each other.

Back

generate row number in SQL Without ROWNUM

Front

Generating a row number - that is a running sequence of numbers for each row is not easy using plain SQL. In fact, the method I am going to show below is not very generic either. This method only works if there is at least one unique column in the table. This method will also work if there is no single unique column, but collection of columns that is unique.

Back

types of join and explain each

Front

Right Join. Right join return rows which are common between the tables and all rows of Right hand side table. Simply, it returns all the rows from the right hand side table even though there are no matches in the left hand side table. Left Join. Left join return rows which are common between the tables and all rows of Left hand side table. Simply, it returns all the rows from Left hand side table even though there are no matches in the Right hand side table. Full Join. Full join return rows when there are matching rows in any one of the tables. This means, it returns all the rows from the left hand side table and all the rows from the right hand side table.

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