sql

Thumbnail image for UNPIVOT in Oracle 11G to Select Columns As Rows

UNPIVOT in Oracle 11G to Select Columns As Rows

Sometimes we need to transpose columns as rows in SQL. Oracle 11G onwards, this requirement can be easily implemented using the UNPIVOT clause. A typical scenario:

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Thumbnail image for SQL to Select Rows Conditionally Based On Column Value

SQL to Select Rows Conditionally Based On Column Value

Let’s say a table contains multiple rows for an id. The requirement is to select only one of those rows, based on the value in a "type" column which determines the row’s priority. A typical example is selecting one contact number for a customer, based on contact types.

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Thumbnail image for SQL to Find Master Records with Identical Detail Records

SQL to Find Master Records with Identical Detail Records

The query to identify duplicate records in Oracle in a single table is fairly straightforward. Things get tricky in a two-table (master-detail) scenario, in which the master table holds the header information (id, name, etc) and the detail table has sets of values associated with the master records. What if we need to find those […]

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Replace Duplicate Ids with Unique Values

Replace Duplicates in ID Column with Unique Values

A common scenario: in the development environment, a table’s ID column is missing its unique key constraint, and developers have been creating their own data for testing. Suddenly someone realizes that the ID column has duplicates. The DBA is summoned. "Enable the unique key", the DBA is told, "but do not delete our data." Can […]

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Thumbnail image for The Curious Case of The Missing ORA-00904

The Curious Case of The Missing ORA-00904

Here’s a puzzle for you to solve. You’re given these two tables – empl and dept – in which column emp_dept_id of table empl references dept_id of table dept. Note that the foreign key column names in the two tables is not identical. It’s called dept_id in table dept, emp_dept_id in table empl.

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Thumbnail image for Flatten Hierarchical Data using a Single SQL

Flatten Hierarchical Data using a Single SQL

Oracle has some neat ways of handling hierarchical data. A while back I wrote about how Oracle SQL can find the entire tree of data above or below a node in a hierarchy using the CONNECT BY clause. What if the requirement is to flatten hierarchical data? For example, given a table containing the employee […]

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Thumbnail image for LISTAGG for Converting Rows to Comma-Separated String

LISTAGG for Converting Rows to Comma-Separated String

In an earlier post, we saw how Oracle SQL can be used to transpose data from rows to columns. It is a similar, though simpler, exercise to transpose data from rows to a comma-seperated list.

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Thumbnail image for Stop Overlapping Ranges in a Table

Stop Overlapping Ranges in a Table

Range-based definition of values is a common business scenario, as in the case of mortgage pre-payment penalty charges depending on "pre-payment amount" slabs, or incentives for a dealer depending on "business generated" slabs. A necessary validation is such cases is that the slabs should have no overlaps: each discrete value should match no more than […]

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Thumbnail image for Using LEAD/LAG to Place a Value within Extents

Using LEAD/LAG to Place a Value within Extents

Take a table with the structure {<Category>, <Extent>}– say, a GRADE_MASTER table with two columns: {GRADE_CODE, SCORE_UPTO}. For examination scores in the range 0-100, GRADE_MASTER specifies the extent of score up to which a certain grade applies. To find the grade for a given the examination score, the SQL needs to compare the EXTENT values […]

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Thumbnail image for How to Delete Old Data from the Entire Schema

How to Delete Old Data from the Entire Schema

We may want to slim down a database schema, pruning old data from all tables, for a number of reasons. Let’s say to set up the test environment for a data migration project, a replica of the legacy database schema has been created. The legacy database schema has millions of records spanning multiple years, but […]

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