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SQLStructured Query Language
SQL
● Define the database
● Query data
● Manipulate data
● Add more data
Databaseapplicationsapplicationsapplications
reporting
administrationSQL
SQL
SQL
reporting
SQL
SQL
SQL
SQL 101
● Define the database
● Query data
● Manipulate data
● Add more data
Database vs. Spreadsheets
● Each table is like an Excel sheet
● Structure is much more important
● Calculations are not* part of the table
Tables
● Divided in rows or records● And divided in columns or fields
Columns describe the characteristics or quantities of what is described in each row, but never* the other way around.
Querying
vegetables
id name color stock_count
1 Apple Red 10
2 Tomato Red 2
3 Banana Yellow 4
4 Avocado Green 15
vegetables
id name color stock_count
1 Apple Red 10
2 Tomato Red 2
3 Banana Yellow 4
4 Avocado Green 15
Sentences and clauses
“Return observations on the following characteristics, from the following tables, where the following criteria is met, organized in the following way.”
The SELECT statement
“Return observations on the following characteristics, from the following tables, where the following criteria is met.”
The SELECT statement
“Return the name and color, from the vegetables table, where the stock count is more than 4.”
The SELECT statement
SELECT the name and color, FROM the vegetables table, WHERE the stock count is more than 4.
The SELECT statement
SELECT the name and color, FROM the vegetables tableWHERE the stock count is more than 4.
The SELECT statement
SELECT name, colorFROM vegetablesWHERE stock count > 4.
The SELECT statement
SELECT "name", "color"FROM "vegetables"WHERE "stock_count" > 4;
vegetables
id name color stock_count
1 Apple Red 10
2 Tomato Red 2
3 Banana Yellow 4
4 Avocado Green 15
Result
name color
Apple Red
Avocado Green
Combining expressions
SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4 AND "color" = 'Red';
vegetables
id name color stock_count
1 Apple Red 10
2 Tomato Red 2
3 Banana Yellow 4
4 Avocado Green 15
Result
id color
1 Red
More combinations
SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4
AND ("color" = 'Red' OR "name" = 'Pear');
SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4
AND ("color" = 'Red' OR "name" = 'Pear') ;
More combinations
SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4
AND ("color" = 'Red' OR "name" = 'Pear') ;
More combinations
More combinations
SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4
AND ("color" = 'Red' OR "name" = 'Pear') ;
Result
id color
1 Red
Other comparisons● Column values can be compared to literals or to other
columns, using:○ =○ >○ <○ >=○ <=○ != ○ some specialized ones for dates, etc.
Sentences and clauses
“Return observations on the following characteristics, from the following tables, where the following criteria is met, organized in the following way.”
Ordering results
SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4
ORDER BY
"id";
vegetables
id name color stock_count
1 Apple Red 10
2 Tomato Red 2
3 Banana Yellow 4
4 Avocado Green 15
Result
name color
Avocado Green
Apple Red
Limiting the result set
SELECT
* --Everything
FROM
"vegetables"
ORDER BY
"stock_count" DESC
LIMIT
2;
Result
id name color stock_count
4 Avocado Green 15
1 Apple Red 10
Top 10 SKUs by stock level
Chartio example
Write a query for a chart (table) that returns the name of the top 10 customers
by qty. of boxes, whose level is either gold or bronze.
DIY
Recap...● Identifiers go between double quotes, "color".
● Text goes between simple quotes, like 'Red'.
● Expressions combine with AND, OR, and NOT.
● We can ORDER BY any column, ASC or DESC.
● We can LIMIT the amount of rows we get.
More than one table
Joining
Sets, the cartesian product
Car
Truck
Van
Red
Green
Blue
color vehicle
Sets, the cartesian product
Car
Truck
Van
Red
Green
Blue
Red Car Red VanRed Truck Blue CarBlue Van Blue Truck...
color vehiclePRODUCT
Two tables
id name color
1 Jacket 2
2 Sweater 1
3 Shirt 1
id name
1 Red
2 Green
3 Blue
color item
Two tables
id name color
1 Jacket 2
2 Sweater 1
3 Shirt 1
id name
1 Red
2 Green
3 Blue
color item
Two tables
id name color
1 Jacket 2
2 Sweater 1
3 Shirt 1
id name
1 Red
2 Green
3 Blue
color item
SELECT
*
FROM
"item"
JOIN
"color";
The JOIN clause
Two tables product
id name id name color
1 Red 1 Jacket 2
1 Red 2 Sweater 1
1 Red 3 Shirt 1
2 Green 1 Jacket 2
2 Green 2 Sweater 1
2 Green 3 Shirt 1
3 Blue 1 Jacket 2
3 Blue 2 Sweater 1
3 Blue 3 Shirt 1
item X color
Two tables, also quite useless
id name color
1 Jacket 2
2 Sweater 1
3 Shirt 1
id name
1 Red
2 Green
3 Blue
color item
Two tables, product
id name id name color
1 Red 1 Jacket 2
1 Red 2 Sweater 1
1 Red 3 Shirt 1
2 Green 1 Jacket 2
2 Green 2 Sweater 1
2 Green 3 Shirt 1
3 Blue 1 Jacket 2
3 Blue 2 Sweater 1
3 Blue 3 Shirt 1
item X color
Two tables, restricted product
id name id name color
1 Red 1 Jacket 2
1 Red 2 Sweater 1
1 Red 3 Shirt 1
2 Green 1 Jacket 2
2 Green 2 Sweater 1
2 Green 3 Shirt 1
3 Blue 1 Jacket 2
3 Blue 2 Sweater 1
3 Blue 3 Shirt 1
item X color, only WHERE the color "id" equals "color"
Two tables, restricted productitem X color, only WHERE the color "id" equals "color"
id name id name color
1 Red 2 Sweater 1
1 Red 3 Shirt 1
2 Green 1 Jacket 2
Only some columnsitem X color product, only WHERE the color "id" equals the item "color",
SELECTing only some columns.
name name
Sweater Red
Shirt Red
Jacket Green
SELECT
"item"."name",
"color"."name"
FROM
"item"
JOIN
"color" ON "item"."color" = "color"."id";
The JOIN clause, ON
Name name what?
name name
Sweater Red
Shirt Red
Jacket Green
SELECT
"item"."name" AS "Item Name",
"color"."name" AS "Color"
FROM
"item"
JOIN
"color" ON "item"."color" = "color"."id";
Aliases, AS
Better
Item name Color
Sweater Red
Shirt Red
Jacket Green
SELECT
i."name" AS "Item Name",
c."name" AS "Color"
FROM
"item" AS i
JOIN
"color" AS c ON i."color" = c."id";
Aliases, AS also for tables
The personal shopper names of thelast 20 signups, without repeating.
Chartio example
Write a query for a chart (table) that returns the name of the top 10 customers
by qty. of boxes, whose level is either gold or bronze. Also include the name of
their PS.
DIY
Summing, counting, averaging multiple rows into one.
Next: Aggregation
Iván Stepaniuk@istepaniuk
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