diff --git a/Chapter_09/Chapter_09.sql b/Chapter_09/Chapter_09.sql
index c4e61d3..06eea5d 100644
--- a/Chapter_09/Chapter_09.sql
+++ b/Chapter_09/Chapter_09.sql
@@ -1,305 +1,343 @@
--- FIRST EDITION FILE; IGNORE
-
-
-
-
-
---------------------------------------------------------------
--- Practical SQL: A Beginner's Guide to Storytelling with Data
+---------------------------------------------------------------------------
+-- Practical SQL: A Beginner's Guide to Storytelling with Data, 2nd Edition
 -- by Anthony DeBarros
 
 -- Chapter 9 Code Examples
---------------------------------------------------------------
+----------------------------------------------------------------------------
 
--- Listing 9-1: Creating and filling the 2014 Public Libraries Survey table
+-- Listing 9-1: Creating and filling the 2018 Public Libraries Survey table
 
-CREATE TABLE pls_fy2014_pupld14a (
-    stabr varchar(2) NOT NULL,
-    fscskey varchar(6) CONSTRAINT fscskey2014_key PRIMARY KEY,
-    libid varchar(20) NOT NULL,
-    libname varchar(100) NOT NULL,
-    obereg varchar(2) NOT NULL,
-    rstatus integer NOT NULL,
-    statstru varchar(2) NOT NULL,
-    statname varchar(2) NOT NULL,
-    stataddr varchar(2) NOT NULL,
-    longitud numeric(10,7) NOT NULL,
-    latitude numeric(10,7) NOT NULL,
-    fipsst varchar(2) NOT NULL,
-    fipsco varchar(3) NOT NULL,
-    address varchar(35) NOT NULL,
-    city varchar(20) NOT NULL,
-    zip varchar(5) NOT NULL,
-    zip4 varchar(4) NOT NULL,
-    cnty varchar(20) NOT NULL,
-    phone varchar(10) NOT NULL,
-    c_relatn varchar(2) NOT NULL,
-    c_legbas varchar(2) NOT NULL,
-    c_admin varchar(2) NOT NULL,
-    geocode varchar(3) NOT NULL,
-    lsabound varchar(1) NOT NULL,
-    startdat varchar(10),
-    enddate varchar(10),
+CREATE TABLE pls_fy2018_libraries (
+    stabr text NOT NULL,
+    fscskey text CONSTRAINT fscskey_2018_pkey PRIMARY KEY,
+    libid text NOT NULL,
+    libname text NOT NULL,
+    address text NOT NULL,
+    city text NOT NULL,
+    zip text NOT NULL,
+    county text NOT NULL,
+    phone text NOT NULL,
+    c_relatn text NOT NULL,
+    c_legbas text NOT NULL,
+    c_admin text NOT NULL,
+    c_fscs text NOT NULL,
+    geocode text NOT NULL,
+    lsabound text NOT NULL,
+    startdate text NOT NULL,
+    enddate text NOT NULL,
     popu_lsa integer NOT NULL,
+    popu_und integer NOT NULL,
     centlib integer NOT NULL,
     branlib integer NOT NULL,
     bkmob integer NOT NULL,
-    master numeric(8,2) NOT NULL,
-    libraria numeric(8,2) NOT NULL,
     totstaff numeric(8,2) NOT NULL,
-    locgvt integer NOT NULL,
-    stgvt integer NOT NULL,
-    fedgvt integer NOT NULL,
-    totincm integer NOT NULL,
-    salaries integer,
-    benefit integer,
-    staffexp integer,
-    prmatexp integer NOT NULL,
-    elmatexp integer NOT NULL,
-    totexpco integer NOT NULL,
-    totopexp integer NOT NULL,
-    lcap_rev integer NOT NULL,
-    scap_rev integer NOT NULL,
-    fcap_rev integer NOT NULL,
-    cap_rev integer NOT NULL,
-    capital integer NOT NULL,
     bkvol integer NOT NULL,
     ebook integer NOT NULL,
     audio_ph integer NOT NULL,
-    audio_dl float NOT NULL,
+    audio_dl integer NOT NULL,
     video_ph integer NOT NULL,
-    video_dl float NOT NULL,
-    databases integer NOT NULL,
+    video_dl integer NOT NULL,
+    ec_lo_ot integer NOT NULL,
     subscrip integer NOT NULL,
     hrs_open integer NOT NULL,
     visits integer NOT NULL,
-    referenc integer NOT NULL,
+    reference integer NOT NULL,
     regbor integer NOT NULL,
     totcir integer NOT NULL,
     kidcircl integer NOT NULL,
-    elmatcir integer NOT NULL,
-    loanto integer NOT NULL,
-    loanfm integer NOT NULL,
     totpro integer NOT NULL,
-    totatten integer NOT NULL,
     gpterms integer NOT NULL,
     pitusr integer NOT NULL,
     wifisess integer NOT NULL,
-    yr_sub integer NOT NULL
+    obereg text NOT NULL,
+    statstru text NOT NULL,
+    statname text NOT NULL,
+    stataddr text NOT NULL,
+    longitude numeric(10,7) NOT NULL,
+    latitude numeric(10,7) NOT NULL
 );
 
-CREATE INDEX libname2014_idx ON pls_fy2014_pupld14a (libname);
-CREATE INDEX stabr2014_idx ON pls_fy2014_pupld14a (stabr);
-CREATE INDEX city2014_idx ON pls_fy2014_pupld14a (city);
-CREATE INDEX visits2014_idx ON pls_fy2014_pupld14a (visits);
-
-COPY pls_fy2014_pupld14a
-FROM 'C:\YourDirectory\pls_fy2014_pupld14a.csv'
+COPY pls_fy2018_libraries
+-- FROM 'C:\YourDirectory\pls_fy2018_libraries.csv'
+from '/Users/adebarros/Dropbox/DataMonky/Book-Writing/PracticalSQL_2e/Code-Repo/Chapter_09/pls_fy2018_libraries.csv'
 WITH (FORMAT CSV, HEADER);
 
--- Listing 9-2: Creating and filling the 2009 Public Libraries Survey table
+CREATE INDEX libname_2018_idx ON pls_fy2018_libraries (libname);
 
-CREATE TABLE pls_fy2009_pupld09a (
-    stabr varchar(2) NOT NULL,
-    fscskey varchar(6) CONSTRAINT fscskey2009_key PRIMARY KEY,
-    libid varchar(20) NOT NULL,
-    libname varchar(100) NOT NULL,
-    address varchar(35) NOT NULL,
-    city varchar(20) NOT NULL,
-    zip varchar(5) NOT NULL,
-    zip4 varchar(4) NOT NULL,
-    cnty varchar(20) NOT NULL,
-    phone varchar(10) NOT NULL,
-    c_relatn varchar(2) NOT NULL,
-    c_legbas varchar(2) NOT NULL,
-    c_admin varchar(2) NOT NULL,
-    geocode varchar(3) NOT NULL,
-    lsabound varchar(1) NOT NULL,
-    startdat varchar(10),
-    enddate varchar(10),
+-- Listing 9-2: Creating and filling the 2017 and 2016 Public Libraries Survey tables
+
+CREATE TABLE pls_fy2017_libraries (
+    stabr text NOT NULL,
+    fscskey text CONSTRAINT fscskey_17_pkey PRIMARY KEY,
+    libid text NOT NULL,
+    libname text NOT NULL,
+    address text NOT NULL,
+    city text NOT NULL,
+    zip text NOT NULL,
+    county text NOT NULL,
+    phone text NOT NULL,
+    c_relatn text NOT NULL,
+    c_legbas text NOT NULL,
+    c_admin text NOT NULL,
+    c_fscs text NOT NULL,
+    geocode text NOT NULL,
+    lsabound text NOT NULL,
+    startdate text NOT NULL,
+    enddate text NOT NULL,
     popu_lsa integer NOT NULL,
+    popu_und integer NOT NULL,
     centlib integer NOT NULL,
     branlib integer NOT NULL,
     bkmob integer NOT NULL,
-    master numeric(8,2) NOT NULL,
-    libraria numeric(8,2) NOT NULL,
     totstaff numeric(8,2) NOT NULL,
-    locgvt integer NOT NULL,
-    stgvt integer NOT NULL,
-    fedgvt integer NOT NULL,
-    totincm integer NOT NULL,
-    salaries integer,
-    benefit integer,
-    staffexp integer,
-    prmatexp integer NOT NULL,
-    elmatexp integer NOT NULL,
-    totexpco integer NOT NULL,
-    totopexp integer NOT NULL,
-    lcap_rev integer NOT NULL,
-    scap_rev integer NOT NULL,
-    fcap_rev integer NOT NULL,
-    cap_rev integer NOT NULL,
-    capital integer NOT NULL,
     bkvol integer NOT NULL,
     ebook integer NOT NULL,
-    audio integer NOT NULL,
-    video integer NOT NULL,
-    databases integer NOT NULL,
+    audio_ph integer NOT NULL,
+    audio_dl integer NOT NULL,
+    video_ph integer NOT NULL,
+    video_dl integer NOT NULL,
+    ec_lo_ot integer NOT NULL,
     subscrip integer NOT NULL,
     hrs_open integer NOT NULL,
     visits integer NOT NULL,
-    referenc integer NOT NULL,
+    reference integer NOT NULL,
     regbor integer NOT NULL,
     totcir integer NOT NULL,
     kidcircl integer NOT NULL,
-    loanto integer NOT NULL,
-    loanfm integer NOT NULL,
     totpro integer NOT NULL,
-    totatten integer NOT NULL,
     gpterms integer NOT NULL,
     pitusr integer NOT NULL,
-    yr_sub integer NOT NULL,
-    obereg varchar(2) NOT NULL,
-    rstatus integer NOT NULL,
-    statstru varchar(2) NOT NULL,
-    statname varchar(2) NOT NULL,
-    stataddr varchar(2) NOT NULL,
-    longitud numeric(10,7) NOT NULL,
-    latitude numeric(10,7) NOT NULL,
-    fipsst varchar(2) NOT NULL,
-    fipsco varchar(3) NOT NULL
+    wifisess integer NOT NULL,
+    obereg text NOT NULL,
+    statstru text NOT NULL,
+    statname text NOT NULL,
+    stataddr text NOT NULL,
+    longitude numeric(10,7) NOT NULL,
+    latitude numeric(10,7) NOT NULL
 );
 
-CREATE INDEX libname2009_idx ON pls_fy2009_pupld09a (libname);
-CREATE INDEX stabr2009_idx ON pls_fy2009_pupld09a (stabr);
-CREATE INDEX city2009_idx ON pls_fy2009_pupld09a (city);
-CREATE INDEX visits2009_idx ON pls_fy2009_pupld09a (visits);
+CREATE TABLE pls_fy2016_libraries (
+    stabr text NOT NULL,
+    fscskey text CONSTRAINT fscskey_16_pkey PRIMARY KEY,
+    libid text NOT NULL,
+    libname text NOT NULL,
+    address text NOT NULL,
+    city text NOT NULL,
+    zip text NOT NULL,
+    county text NOT NULL,
+    phone text NOT NULL,
+    c_relatn text NOT NULL,
+    c_legbas text NOT NULL,
+    c_admin text NOT NULL,
+    c_fscs text NOT NULL,
+    geocode text NOT NULL,
+    lsabound text NOT NULL,
+    startdate text NOT NULL,
+    enddate text NOT NULL,
+    popu_lsa integer NOT NULL,
+    popu_und integer NOT NULL,
+    centlib integer NOT NULL,
+    branlib integer NOT NULL,
+    bkmob integer NOT NULL,
+    totstaff numeric(8,2) NOT NULL,
+    bkvol integer NOT NULL,
+    ebook integer NOT NULL,
+    audio_ph integer NOT NULL,
+    audio_dl integer NOT NULL,
+    video_ph integer NOT NULL,
+    video_dl integer NOT NULL,
+    ec_lo_ot integer NOT NULL,
+    subscrip integer NOT NULL,
+    hrs_open integer NOT NULL,
+    visits integer NOT NULL,
+    reference integer NOT NULL,
+    regbor integer NOT NULL,
+    totcir integer NOT NULL,
+    kidcircl integer NOT NULL,
+    totpro integer NOT NULL,
+    gpterms integer NOT NULL,
+    pitusr integer NOT NULL,
+    wifisess integer NOT NULL,
+    obereg text NOT NULL,
+    statstru text NOT NULL,
+    statname text NOT NULL,
+    stataddr text NOT NULL,
+    longitude numeric(10,7) NOT NULL,
+    latitude numeric(10,7) NOT NULL
+);
 
-COPY pls_fy2009_pupld09a
-FROM 'C:\YourDirectory\pls_fy2009_pupld09a.csv'
+COPY pls_fy2017_libraries
+-- FROM 'C:\YourDirectory\pls_fy2017_libraries.csv'
+from '/Users/adebarros/Dropbox/DataMonky/Book-Writing/PracticalSQL_2e/Code-Repo/Chapter_09/pls_fy2017_libraries.csv'
 WITH (FORMAT CSV, HEADER);
 
+COPY pls_fy2016_libraries
+-- FROM 'C:\YourDirectory\pls_fy2016_libraries.csv'
+from '/Users/adebarros/Dropbox/DataMonky/Book-Writing/PracticalSQL_2e/Code-Repo/Chapter_09/pls_fy2016_libraries.csv'
+WITH (FORMAT CSV, HEADER);
+
+CREATE INDEX libname_2017_idx ON pls_fy2017_libraries (libname);
+CREATE INDEX libname_2016_idx ON pls_fy2016_libraries (libname);
+
+
 -- Listing 9-3: Using count() for table row counts
 
 SELECT count(*)
-FROM pls_fy2014_pupld14a;
+FROM pls_fy2018_libraries;
 
 SELECT count(*)
-FROM pls_fy2009_pupld09a;
+FROM pls_fy2017_libraries;
+
+SELECT count(*)
+FROM pls_fy2016_libraries;
 
 -- Listing 9-4: Using count() for the number of values in a column
 
-SELECT count(salaries)
-FROM pls_fy2014_pupld14a;
+SELECT count(phone)
+FROM pls_fy2018_libraries;
 
 -- Listing 9-5: Using count() for the number of distinct values in a column
 
 SELECT count(libname)
-FROM pls_fy2014_pupld14a;
+FROM pls_fy2018_libraries;
 
 SELECT count(DISTINCT libname)
-FROM pls_fy2014_pupld14a;
+FROM pls_fy2018_libraries;
 
 -- Bonus: find duplicate libnames
 SELECT libname, count(libname)
-FROM pls_fy2014_pupld14a
+FROM pls_fy2018_libraries
 GROUP BY libname
 ORDER BY count(libname) DESC;
 
 -- Bonus: see location of every Oxford Public Library
 SELECT libname, city, stabr
-FROM pls_fy2014_pupld14a
+FROM pls_fy2018_libraries
 WHERE libname = 'OXFORD PUBLIC LIBRARY';
 
 -- Listing 9-6: Finding the most and fewest visits using max() and min()
 SELECT max(visits), min(visits)
-FROM pls_fy2014_pupld14a;
+FROM pls_fy2018_libraries;
 
 -- Listing 9-7: Using GROUP BY on the stabr column
 
--- There are 56 in 2014.
+-- There are 55 in 2018.
 SELECT stabr
-FROM pls_fy2014_pupld14a
+FROM pls_fy2018_libraries
 GROUP BY stabr
 ORDER BY stabr;
 
--- Bonus: there are 55 in 2009.
+-- Bonus: there are 54 in 2017.
 SELECT stabr
-FROM pls_fy2009_pupld09a
+FROM pls_fy2017_libraries
 GROUP BY stabr
 ORDER BY stabr;
 
 -- Listing 9-8: Using GROUP BY on the city and stabr columns
 
 SELECT city, stabr
-FROM pls_fy2014_pupld14a
+FROM pls_fy2018_libraries
 GROUP BY city, stabr
 ORDER BY city, stabr;
 
--- Bonus: We can count some of the combos
+-- Bonus: We can count the combos
 SELECT city, stabr, count(*)
-FROM pls_fy2014_pupld14a
+FROM pls_fy2018_libraries
 GROUP BY city, stabr
 ORDER BY count(*) DESC;
 
--- Listing 9-9: GROUP BY with count() on the stabr column
+-- Listing 9-9: Using GROUP BY with count() on the stabr column
 
 SELECT stabr, count(*)
-FROM pls_fy2014_pupld14a
+FROM pls_fy2018_libraries
 GROUP BY stabr
 ORDER BY count(*) DESC;
 
 -- Listing 9-10: GROUP BY with count() on the stabr and stataddr columns
 
 SELECT stabr, stataddr, count(*)
-FROM pls_fy2014_pupld14a
+FROM pls_fy2018_libraries
 GROUP BY stabr, stataddr
-ORDER BY stabr ASC, count(*) DESC;
+ORDER BY stabr, stataddr;
 
 -- Listing 9-11: Using the sum() aggregate function to total visits to
 -- libraries in 2014 and 2009
 
--- 2014
-SELECT sum(visits) AS visits_2014
-FROM pls_fy2014_pupld14a
+-- 2018
+SELECT sum(visits) AS visits_2018
+FROM pls_fy2018_libraries
 WHERE visits >= 0;
 
--- 2009
-SELECT sum(visits) AS visits_2009
-FROM pls_fy2009_pupld09a
+-- 2017
+SELECT sum(visits) AS visits_2017
+FROM pls_fy2017_libraries
 WHERE visits >= 0;
 
--- Listing 9-12: Using sum() to total visits on joined 2014 and 2009 library tables
+-- 2016
+SELECT sum(visits) AS visits_2016
+FROM pls_fy2016_libraries
+WHERE visits >= 0;
+
+-- Listing 9-12: Using sum() to total visits on joined 2018, 2017, and 2016 tables
+
+SELECT sum(pls18.visits) AS visits_2018,
+       sum(pls17.visits) AS visits_2017,
+       sum(pls16.visits) AS visits_2016
+FROM pls_fy2018_libraries pls18
+       JOIN pls_fy2017_libraries pls17 ON pls18.fscskey = pls17.fscskey
+       JOIN pls_fy2016_libraries pls16 ON pls18.fscskey = pls16.fscskey
+WHERE pls18.visits >= 0
+       AND pls17.visits >= 0
+       AND pls16.visits >= 0;
+
+-- Bonus: summing wifi sessions
+SELECT sum(pls18.wifisess) AS wifi_2018,
+       sum(pls17.wifisess) AS wifi_2017,
+       sum(pls16.wifisess) AS wifi_2016
+FROM pls_fy2018_libraries pls18
+       JOIN pls_fy2017_libraries pls17 ON pls18.fscskey = pls17.fscskey
+       JOIN pls_fy2016_libraries pls16 ON pls18.fscskey = pls16.fscskey
+WHERE pls18.wifisess >= 0
+       AND pls17.wifisess >= 0
+       AND pls16.wifisess >= 0;
 
-SELECT sum(pls14.visits) AS visits_2014,
-       sum(pls09.visits) AS visits_2009
-FROM pls_fy2014_pupld14a pls14 JOIN pls_fy2009_pupld09a pls09
-ON pls14.fscskey = pls09.fscskey
-WHERE pls14.visits >= 0 AND pls09.visits >= 0;
 
 -- Listing 9-13: Using GROUP BY to track percent change in library visits by state
 
-SELECT pls14.stabr,
-       sum(pls14.visits) AS visits_2014,
-       sum(pls09.visits) AS visits_2009,
-       round( (CAST(sum(pls14.visits) AS decimal(10,1)) - sum(pls09.visits)) /
-                    sum(pls09.visits) * 100, 2 ) AS pct_change
-FROM pls_fy2014_pupld14a pls14 JOIN pls_fy2009_pupld09a pls09
-ON pls14.fscskey = pls09.fscskey
-WHERE pls14.visits >= 0 AND pls09.visits >= 0
-GROUP BY pls14.stabr
-ORDER BY pct_change DESC;
+SELECT pls18.stabr,
+       sum(pls18.visits) AS visits_2018,
+       sum(pls17.visits) AS visits_2017,
+       sum(pls16.visits) AS visits_2016,
+       round( (sum(pls18.visits::numeric) - sum(pls17.visits)) /
+            sum(pls17.visits) * 100, 1 ) AS chg_2018_17,
+       round( (sum(pls17.visits::numeric) - sum(pls16.visits)) /
+            sum(pls16.visits) * 100, 1 ) AS chg_2017_16
+FROM pls_fy2018_libraries pls18
+       JOIN pls_fy2017_libraries pls17 ON pls18.fscskey = pls17.fscskey
+       JOIN pls_fy2016_libraries pls16 ON pls18.fscskey = pls16.fscskey
+WHERE pls18.visits >= 0
+       AND pls17.visits >= 0
+       AND pls16.visits >= 0
+GROUP BY pls18.stabr
+ORDER BY chg_2018_17 DESC;
 
 -- Listing 9-14: Using HAVING to filter the results of an aggregate query
 
-SELECT pls14.stabr,
-       sum(pls14.visits) AS visits_2014,
-       sum(pls09.visits) AS visits_2009,
-       round( (CAST(sum(pls14.visits) AS decimal(10,1)) - sum(pls09.visits)) /
-                    sum(pls09.visits) * 100, 2 ) AS pct_change
-FROM pls_fy2014_pupld14a pls14 JOIN pls_fy2009_pupld09a pls09
-ON pls14.fscskey = pls09.fscskey
-WHERE pls14.visits >= 0 AND pls09.visits >= 0
-GROUP BY pls14.stabr
-HAVING sum(pls14.visits) > 50000000
-ORDER BY pct_change DESC;
+SELECT pls18.stabr,
+       sum(pls18.visits) AS visits_2018,
+       sum(pls17.visits) AS visits_2017,
+       sum(pls16.visits) AS visits_2016,
+       round( (sum(pls18.visits::numeric) - sum(pls17.visits)) /
+            sum(pls17.visits) * 100, 1 ) AS chg_2018_17,
+       round( (sum(pls17.visits::numeric) - sum(pls16.visits)) /
+            sum(pls16.visits) * 100, 1 ) AS chg_2017_16
+FROM pls_fy2018_libraries pls18
+       JOIN pls_fy2017_libraries pls17 ON pls18.fscskey = pls17.fscskey
+       JOIN pls_fy2016_libraries pls16 ON pls18.fscskey = pls16.fscskey
+WHERE pls18.visits >= 0
+       AND pls17.visits >= 0
+       AND pls16.visits >= 0
+GROUP BY pls18.stabr
+HAVING sum(pls18.visits) > 50000000
+ORDER BY chg_2018_17 DESC;
+
diff --git a/Try_It_Yourself/Try_It_Yourself.sql b/Try_It_Yourself/Try_It_Yourself.sql
index 1eb1350..e33e1f6 100644
--- a/Try_It_Yourself/Try_It_Yourself.sql
+++ b/Try_It_Yourself/Try_It_Yourself.sql
@@ -470,5 +470,140 @@ CREATE TABLE songs (
 -- to perform many queries that include date ranges.
 
 
+----------------------------------------------------------------------------
+-- Chapter 9: Extracting Information by Grouping and Summarizing
+----------------------------------------------------------------------------
 
+-- 1. We saw that library visits have declined recently in most places. But 
+-- what is the pattern in library employment? All three library survey tables
+-- contain the column totstaff, which is the number of paid full-time equivalent
+-- employees. Modify the code in Listings 9-13 and 9-14 to calculate the 
+-- percent change in the sum of the column over time, examining all states as 
+-- well as states with the most visitors. Watch out for negative values!
 
+-- Answer (all states):
+
+SELECT pls18.stabr,
+       sum(pls18.totstaff) AS totstaff_2018,
+       sum(pls17.totstaff) AS totstaff_2017,
+       sum(pls16.totstaff) AS totstaff_2016,
+       round( (sum(pls18.totstaff::numeric) - sum(pls17.totstaff)) /
+            sum(pls17.totstaff) * 100, 1 ) AS chg_2018_17,
+       round( (sum(pls17.totstaff::numeric) - sum(pls16.totstaff)) /
+            sum(pls16.hrs_open) * 100, 1 ) AS chg_2017_16
+FROM pls_fy2018_libraries pls18
+       JOIN pls_fy2017_libraries pls17 ON pls18.fscskey = pls17.fscskey
+       JOIN pls_fy2016_libraries pls16 ON pls18.fscskey = pls16.fscskey
+WHERE pls18.totstaff >= 0
+       AND pls17.totstaff >= 0
+       AND pls16.totstaff >= 0
+GROUP BY pls18.stabr
+ORDER BY chg_2018_17 DESC;
+
+-- Answer (filtered with HAVING):
+
+SELECT pls18.stabr,
+       sum(pls18.totstaff) AS totstaff_2018,
+       sum(pls17.totstaff) AS totstaff_2017,
+       sum(pls16.totstaff) AS totstaff_2016,
+       round( (sum(pls18.totstaff::numeric) - sum(pls17.totstaff)) /
+            sum(pls17.totstaff) * 100, 1 ) AS chg_2018_17,
+       round( (sum(pls17.totstaff::numeric) - sum(pls16.totstaff)) /
+            sum(pls16.hrs_open) * 100, 1 ) AS chg_2017_16
+FROM pls_fy2018_libraries pls18
+       JOIN pls_fy2017_libraries pls17 ON pls18.fscskey = pls17.fscskey
+       JOIN pls_fy2016_libraries pls16 ON pls18.fscskey = pls16.fscskey
+WHERE pls18.totstaff >= 0
+       AND pls17.totstaff >= 0
+       AND pls16.totstaff >= 0
+GROUP BY pls18.stabr
+HAVING sum(pls18.visits) > 50000000
+ORDER BY chg_2018_17 DESC;
+
+-- 2. The library survey tables contain a column called obereg, a two-digit
+-- Bureau of Economic Analysis Code that classifies each library agency
+-- according to a region of the United States, such as New England, Rocky
+-- Mountains, and so on. Just as we calculated the percent change in visits
+-- grouped by state, do the same to group percent changes in visits by U.S.
+-- region using obereg. Consult the survey documentation to find the meaning
+-- of each region code. For a bonus challenge, create a table with the obereg
+-- code as the primary key and the region name as text, and join it to the
+-- summary query to group by the region name rather than the code.
+
+-- Answer:
+
+-- a) sum() visits by region.
+    
+SELECT pls18.obereg,
+       sum(pls18.visits) AS visits_2018,
+       sum(pls17.visits) AS visits_2017,
+       sum(pls16.visits) AS visits_2016,
+       round( (sum(pls18.visits::numeric) - sum(pls17.visits)) /
+            sum(pls17.visits) * 100, 1 ) AS chg_2018_17,
+       round( (sum(pls17.visits::numeric) - sum(pls16.visits)) /
+            sum(pls16.visits) * 100, 1 ) AS chg_2017_16
+FROM pls_fy2018_libraries pls18
+       JOIN pls_fy2017_libraries pls17 ON pls18.fscskey = pls17.fscskey
+       JOIN pls_fy2016_libraries pls16 ON pls18.fscskey = pls16.fscskey
+WHERE pls18.visits >= 0
+       AND pls17.visits >= 0
+       AND pls16.visits >= 0
+GROUP BY pls18.obereg
+ORDER BY chg_2018_17 DESC;
+
+-- b) Bonus: creating the regions lookup table and adding it to the query.
+
+CREATE TABLE obereg_codes (
+    obereg text CONSTRAINT obereg_key PRIMARY KEY,
+    region text
+);
+
+INSERT INTO obereg_codes
+VALUES ('01', 'New England (CT ME MA NH RI VT)'),
+       ('02', 'Mid East (DE DC MD NJ NY PA)'),
+       ('03', 'Great Lakes (IL IN MI OH WI)'),
+       ('04', 'Plains (IA KS MN MO NE ND SD)'),
+       ('05', 'Southeast (AL AR FL GA KY LA MS NC SC TN VA WV)'),
+       ('06', 'Soutwest (AZ NM OK TX)'),
+       ('07', 'Rocky Mountains (CO ID MT UT WY)'),
+       ('08', 'Far West (AK CA HI NV OR WA)'),
+       ('09', 'Outlying Areas (AS GU MP PR VI)');
+
+-- sum() visits by region.
+
+SELECT obereg_codes.region,
+       sum(pls18.visits) AS visits_2018,
+       sum(pls17.visits) AS visits_2017,
+       sum(pls16.visits) AS visits_2016,
+       round( (sum(pls18.visits::numeric) - sum(pls17.visits)) /
+            sum(pls17.visits) * 100, 1 ) AS chg_2018_17,
+       round( (sum(pls17.visits::numeric) - sum(pls16.visits)) /
+            sum(pls16.visits) * 100, 1 ) AS chg_2017_16
+FROM pls_fy2018_libraries pls18
+       JOIN pls_fy2017_libraries pls17 ON pls18.fscskey = pls17.fscskey
+       JOIN pls_fy2016_libraries pls16 ON pls18.fscskey = pls16.fscskey
+       JOIN obereg_codes ON pls18.obereg = obereg_codes.obereg
+WHERE pls18.visits >= 0
+       AND pls17.visits >= 0
+       AND pls16.visits >= 0
+GROUP BY obereg_codes.region
+ORDER BY chg_2018_17 DESC;
+
+-- 3. Thinking back to the types of joins you learned in Chapter 7,
+-- which join type will show you all the rows in all three tables,
+-- including those without a match? Write such a query and add an
+-- IS NULL filter in a WHERE clause to show agencies not included
+-- in one or more of the tables.
+
+-- Answer: a FULL OUTER JOIN will show all rows in both tables.
+
+SELECT pls18.libname, pls18.city, pls18.stabr, pls18.statstru, 
+       pls17.libname, pls17.city, pls17.stabr, pls17.statstru, 
+       pls16.libname, pls16.city, pls16.stabr, pls16.statstru
+FROM pls_fy2018_libraries pls18
+       FULL OUTER JOIN pls_fy2017_libraries pls17 ON pls18.fscskey = pls17.fscskey
+       FULL OUTER JOIN pls_fy2016_libraries pls16 ON pls18.fscskey = pls16.fscskey
+WHERE pls16.fscskey IS NULL OR pls17.fscskey IS NULL;
+
+-- Note: The IS NULL statements in the WHERE clause limit results to those
+-- that do not appear in one or more tables.