exercises, answers and "cheat sheet"

NEBC Database Course PostgreSQL Cheat-sheet v2 

December 2008 

For each example, replace mytable and mycol with your own table and column names. The 

new element in each example is highlighted in bold. 

SELECT 

SELECT * from mytable 

SELECT DISTINCT * FROM mytable 

SELECT mycol1, mycol2 FROM table 

Command 

SELECT mycol1 AS foo FROM mytable 

SELECT * FROM mytable ORDER BY mycol1 ASC, mycol2 DESC 

SELECT * FROM mytable WHERE mycol1 = mycol2 

SELECT * FROM mytable WHERE mycol1 = 5 OR 

( mycol1 = 6 AND lower(mycol2) = 'snake' ) 

SELECT * FROM table WHERE mycol1 IS NULL 

get all entries 

get all unique entries 

What it does 

get only columns mycol1 and mycol2 

get mycol1, but call it “foo” in the results 

sort by mycol1 – if 2 rows have the same value here then 

sort these by mycol2 in reverse order 

a simple search condition based on equality 

find all rows where mycol1 is 5, or where mycol1 is 6 but 

also mycol2 contains 'snake', 'Snake', 'sNAke' etc. 

searching for NULL values with 'IS NULL' 

SELECT * FROM mytable WHERE mycol1 IN (3, 4, 5) specify that col1 must be 3, 4 or 5 

SELECT * FROM mytable1 WHERE mycol1 IN 

(SELECT mycol2 FROM mytable2) 

obtain the list of matches from col2 in table2 

SELECT * FROM my table1 t1 WHERE EXISTS 

(SELECT * FROM mytable2 t2 WHERE t2.mycol2 = t1.mycol1) 

SELECT t1.*, t2.* FROM mytable1 t1 INNER JOIN mytable2 t2 ON 

(t1.pk_col = t2.fk_col) 

SELECT t1.*, t2.* FROM mytable1 t1 LEFT OUTER JOIN mytable2 t2 ON 

(t1.pk_col = t2.fk_col) 

SELECT mycol1 as foo, mycol2 AS bar FROM mytable1 

UNION 


same as the previous, but using the EXISTS clause. 

Choose whichever you find easiest! 

inner join, where tables 1 and 2 have a one-to-many 

relationship 

an outer join where columns from table 1 will be 

displayed even if there is no matching entry in table 2 

combine two SELECTS into a single result set – note the 

SELECT keyword appears twice, but the ORDER BY 

affects the whole result. Duplicate rows are not shown. 

Use 'UNION ALL' to show all duplicate rows 

ORDER BY foo 


INTERSECT 

Shows all rows common to two SELECT statements. 

'INTERSECT ALL' will show all duplicate rows 


ORDER BY foo 

SELECT mycol1, sum(mycol2) FROM mytable1 

WHERE mycol3 = 1 

GROUP BY mycol1 HAVING count(*) < 3 

SELECT * INTO mynewtable FROM myoldtable 

group entries in table1 by col1, showing each value of 

col1 as well as the sum of col2 for those rows 

disregard rows where col3 is not 1, and show only results 

with 1 or 2 occurrences of that value of col1 in the table 

Create newtable from oldtable

DELETE, INSERT, UPDATE 


DELETE FROM mytable 

DELETE FROM mytable WHERE mycol1 != 5 

INSERT INTO mytable (mycol1, mycol2) VALUES ('foo', 'bar') 

INSERT INTO mytable2 SELECT mycol1, mycol2 FROM table1 

UPDATE mytable 

What it does 

delete EVERYTHING in a table 

delete by some condition 

insert with specific values 

insert values based on a SELECT statement 

update with WHERE condition 

SET mycol1 = 'foo', mycol2 = mycol2 * mycol2 

WHERE mycol3 IS NOT NULL 

OPERATORS AND FUNCTIONS 


SELECT (mycol1 / mycol2) *100 AS percentage FROM mytable 

SELECT sqrt(mycol1) FROM mytable 

SELECT trunc(mycol1, 3) FROM mytable 

What it does 

perform some arithmetic on the columns (other 

mathematical operators include + and -) 

Select the square root of mycol1 

Truncate the value of mycol3 after 3 decimal points 

SELECT abs(mycol), round(mycol,2), ln(mycol1), sin(mycol), tan(mycol) 

FROM mytable 

SELECT mycol1 || ' and ' || mycol2 FROM mytable 

Absolute value of mycol,round mycol to 2 decimal 

places, natural log mycol,sine of mycol,tangent of mycol 

See documentation for further mathematical functions 

Concatenate mycol1 to mycol2 with the word 'and' 

SELECT substr(mycol1, 2, 3) FROM mytable Select substring of mycol1 starting at character 2, 3 

characters in length 

SELECT upper(mycol1) FROM mytable 

SELECT lower(mycol1) FROM mytable 

SELECT * FROM mytable WHERE mycol1 = 'badger' 

SELECT * FROM mytable WHERE mycol1 != 'badger' 

SELECT * FROM mytable WHERE mycol1 LIKE '%adg%' 

SELECT sum(mycol1), avg(mycol1) FROM mytable 

SELECT count(*) FROM mytable 

SELECT mycol1, sum(mycol2) FROM mytable 

WHERE mycol3 = 1 

GROUP BY mycol1 HAVING count(*) < 3 

SELECT current_timestamp, now() 

SELECT to_char(mydatecol,'DD/MM/YYYY') FROM mytable 

SELECT to_date('200425thJune','YYYYDDthmonth') 

SELECT mydatecol::text FROM mytable1 

Display mycol1 in upper case 

Display mycol1 in lower case 

Select rows where mycol1 exactly matches 'badger' 

Select rows where mycol1 does not match 'badger' 

Select rows where mycol1 contains the string 'adg' (note 

use of '%' wildcard character) 

return both sum and average of mycol1 

Count the number of rows in specified table 

group entries in mytable by mycol1, showing each value 

of mycol1 as well as the sum of mycol2 for those rows 

disregard rows where mycol3 is not 1, and show only 

results with 1 or 2 occurrences of that value of mycol1 

Two ways to display the current date and time 

Select mydatecol in the format 

DD/MM/YYYY(eg.21/04/2008) 

Convert the string '200425thJune' to a date 

Typecast date values explicitly to data type varchar 

SELECT coalesce(mycol1::text, 'No value') FROM mytable1 Display 'No value' if value in mydf -h 

col1 is null 

SELECT CASE WHEN mycol1 = true THEN 'badger' ELSE 'mushroom' END 

FROM mytable1 

If boolean value in mycol1 is true print 'badger' otherwise 

print 'mushroom'

Data Types 

integer,int,int4 

float 

numeric(p,s) 

date 

timestamp 

varchar(n), character varying(n) 

char(n) 

text 

boolean, bool 

Data Type 

Whole number/integer 

Floating point number 

Description 

Exact numeric type with total digits 'p' and digits after 

decimal point 's' 

Calender date 

Date and time 

Variable length character string of max length 'n' 

Fixed length character string of length 'n' 

Variable length character of unlimited length 

A single true or false value (supported values: 

true/false,'t'/'f','true'/'false','y'/'n','yes'/'no','1','0')

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exercises, answers and "cheat sheet"

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