Advanced SQL - Subqueries and Complex Joins Outline for Today: • The URISA Proceedings database - more practice with increasingly complicated SQL queries • Advanced Queries: o Sub-queries: one way to nest or a cascade query is to stick a query in the 'where' clause: e.g., find parcels owned by XXX from that set of parcels that had a fire. This is a powerful way to take advantage of the fact that any SQL query returns a table -which can they be the starting point of another SQL query. o Self-joins: the 'where' clause can become quite complex with many joins and related 'and' and 'or' conditions. But handling 'and' conditions is a little tricky. How can you find papers the use both keyword Y and keyword Z if the table relating papers and keywords only shows one pair at a time? • The zoning variance database o Understanding the schema and rationale for the Boston zoning variance database (which we use later to map them as study spatial patterns as well as to illustrate concepts about distributed databases and community empowerment. o Using the history of zoning database to understand how real databases evolve over time More URISA database Queries • ...from the URISA database* page • Additional notes on SQL*Plus formatting* added to SQL Notes* Advanced Queries: Subqueries A subquery can be nested within a query * Kindly refer to Lecture Notes sectionExample: Find the parcel with the highest estimated loss from a fire SELECT * FROM FIRES WHERE ESTLOSS = (SELECT MAX(ESTLOSS)FROM FIRES);Alternatively, include the subquery as an inline "table" in the FROM clause: SELECT F.* FROM FIRES F,(SELECT MAX(ESTLOSS) MAXLOSSFROM FIRES) MWHERE F.ESTLOSS = M.MAXLOSS; Example: Find the parcels that have not had a fire SELECT * FROM PARCELS WHERE PARCELID NOT IN (SELECT PARCELIDFROM FIRES); or, more efficiently, SELECT * FROM PARCELS P WHERE NOT EXISTS (SELECT NULLFROM FIRES F WHERE P.PARCELID = F.PARCELID); Example: Find the parcels that have not obtained a permit: SELECT * FROM PARCELS WHERE (PID, WPB) NOT IN (SELECT PID, WPBFROM PERMITS); or, more efficiently, SELECT * FROM PARCELS PWHERE NOT EXISTS (SELECT NULLFROM FIRES F WHERE P.PARCELID = F.PARCELID); Advanced Queries: Self-Join A table can be joined to itself Example: Find the paper numbers in the URISA database for papers that use both keyword code 601 AND 602. The following query does not work, because it is not possible for value for a single column in a single row to contain two values at the same time: SELECT PAPER FROM MATCH WHERE CODE = 601 AND CODE = 602;This type of query requires a self-join, which acts as if we had two copies of the MATCH table and are joining them to each other. SELECT M1.PAPER FROM MATCH M1, MATCH M2WHERE M1.PAPER = M2.PAPER AND M1.CODE = 601 AND M2.CODE = 602;If you have trouble imagining the self-join, pretend that we actually created two copies of MATCH, M1 and M2: CREATE TABLE M1 AS SELECT * FROM MATCH;CREATE TABLE M2 AS SELECT * FROM MATCH; Then, we could join M1 and M2: SELECT M1.PAPER FROM M1, M2WHERE M1.PAPER = M2.PAPER AND M1.CODE = 601 AND M2.CODE = 602;The self-join allows us to perform this sort of operation without actually having to copy the table. We can just act as if we had two copies.Now, let's add the titles to the paper numbers: SELECT M1.PAPER, T.TITLE FROM MATCH M1, MATCH M2, TITLES T WHERE M1.PAPER = M2.PAPER AND M1.PAPER = T.PAPER AND M1.CODE = 601 AND M2.CODE = 602; Example: Find the time that passed between a fire on a parcel and all fires occurring within 300 days later on the same parcel SELECT F1.PARCELID, F1.FDATE FIRE1, F2.FDATEFIRE2, F2.FDATE - F1.FDATE INTERVAL FROM FIRES F1, FIRES F2WHERE F1.PARCELID = F2.PARCELID AND F2.FDATE > F1.FDATE AND F2.FDATE <= F1.FDATE + 300; Note that a number of days can be added to a date. The Zoning Variance Database Zoning Variances* Schema of ZONING table (and listing of related lookup tables) SQL examples using zoning variances * Annotated SQL queries of ZONING table 1980 Census data (by Boston NSA)* Schema of 1980 Boston Census data (and related lookup tables) Schema of Decision, Use, NSA, Neighbrhd Lookup Tables* Schema of Lookup tables (second half of Census data web page) Sub-Neighborhood lookup table* The NSA and NEIGHBRHD tables (bottom of Zoning Variance web page) Grouping zoning applicants via 'lookup' tables* Annotated SQL queries illustrating * Kindly refer to Lecture Notes sectionuse of lookup tables to categorize ownership of properties seeking zoning variances. (These topics are the focus of next week's lecture and lab #3.) Zoning Variance Database Evolution Chart * Stages of evolution of the ZONING variance database * Kindly refer to Lecture Notes