How it works

The COUNT function returns the number of input rows that match a specific condition or query SELECT COUNT(*) FROM table_name COUNT - does not consider null values in the column — SELECT COUNT(DISTINCT col) FROM table use above instead of SELECT DISTINCT COUNT(col) FROM table because the first one works and the second does not

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What are Databases ?
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Why Use Databases ?
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spreadsheets vs databases
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From spreadsheets to databases
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Database Platform options
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What is SQL ?
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PgAdmin vs PostgreSQL
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PostgreSQL uses a client and a server, where the server is the process managing the database and accepting connections, whereas the client is what connects to the server and through it interacts with the database
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Roles in postgres are like users and groups in Linux, but combined, since postgres doesn't distringuish between users and groups and instead prefers the term *role*.
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Upon installation, Postgres is set up to use *ident* authentication, meaning that it associates postgres roles with a matching Linux account. If a role exists within postgres, a Linux username with the same name is able to sign in as that role.
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The installation of postgresql creates a user account called postgres that is associated with the default postgres role. In order to use postgres, you can log into that account.
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At first you can run postgres by either switching to the postgres user and running psql (1) or directly running psql as that user (2) 1. sudo -i -u postgres; psql 2. sudo -u postgres psql
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Creating a new role: if you are currently the user postgres, do $ createuser --interactive else $ sudo -u postgres createuser --interactive
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Creating a new database Another assumption that the pstgres authentication system makes by default is that for any role used to log in, that role will have a database with the same name which it can access create a database using createdb $ createdb kolev
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Change a role's password in PostgreSQL $ psql \password <role>
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When restoring a database, remember to toggle the restore option *Data* -> Yes, otherwise you will probably only get empty tables (also check pre-data and post-data if you get an error)
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SELECT - used to query data from a table SELECT col1, col2 FROM table_name User a comma between each column in case you want to query data from multiple columns
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SQL is case insensitive, though by convention SQL keywords are uppercase - usually not great practice to use an * after the SELECT keyword - higher traffic (b/w client & server) - more time to get it done
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character varying(45) - string up to 45 characters (inclusive) In a query, strings are surrounded by single quotes
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SELECT DISTINCT col1, col2 FROM table_name Selects unique row values
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SELECT col1, col2 FROM table_name WHERE conditions
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WHERE statement operators: = - equal > - greater than < - less than >= - greater than or equal to <= - less than or equal to <> or != - not equal AND - logical operator AND OR - logical operator OR
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LIMIT - allows you to limit the number of rows you get back after a query - useful when wanting to get all columns but not all rows SELECT * FROM table LIMIT 1 - goes at the end of a query
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When you query data from a table, PostgreSQL returns the rows in the order that they were inserted into the table ORDER BY - allows you to sort the rows returned from SELECT in ascending or descending order based on criteria specified
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You can also ORDER BY columns that are not selected (in PostgreSQL, not sure about MySQL) Hence it's good practice to always select the columns you're ordering by
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BETWEEN - match a value against a range of values (inclusive) SELECT payment FROM table WHERE amount >= 1 AND amount <= 10 is the same as WHERE amount BETWEEN 1 AND 10
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IN - used with the WHERE caluse to check if a value matches any value in a list (array) of values value IN (val1, val2, val3) can also use NOT IN IN faster than multiple ORs
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Subquery - a query within a query SELECT * from table WHERE value in (SELECT customer_id FROM table)
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Pattern matching using LIKE - CASE SENSITIVE % - any sequence of characters _ - any single character SELECT first_name FROM customer WHERE first_name LIKE 'Jo%' matches any first_name starting with 'Jo'
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Aggregate functions Many Inputs -> One Output AVG - average MAX - maximum MIN - minimum SUM - sum COUNT - number of inputs
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ROUND ROUND(amount, 2) - rounds amount to 2 decimal points
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GROUP BY - divide the rows returned from SELECT into groups SELECT col1, aggregate_func(col2) FROM table GROUP BY col1 GROUP BY without an aggregate function acts like DISTINCT
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GROUP BY + an aggregate function SELECT customer_id, SUM(amount) FROM table GROUP BY customer_id will group up all repeating rows into one and sum their amounts into one
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HAVING - used with the GROUP BY clause to filter out group rows that do not satisfy a specified condition (like WHERE but for GROUP BY) - having, like where but for groups SELECT col1, SUM(col2) FROM table GROUP BY col1 HAVING condition
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The HAVING clause sets the condition for group rows created by the GROUP BY clause after the GROUP BY clause applied while the WHERE clause sets the condition for individual rows before the GROUP BY clause applies
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AS - allows us to rename columns or table selections with an alias SELECT payment as my_payment_column FROM table SELECT payment_id, SUM(amount) AS sum FROM payment GROUP BY payment_id
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JOINS - allow you to relate data from one table to data in other tables 3 types of joins: - INNER JOIN - OUTER JOIN - self-join
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On most SQL engines, you can type JOIN instead of INNER JOIN and it will default to INNER
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SELECT language.name, film.title FROM film INNER JOIN language AS lang ON lang_id = film.language_id
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Practice INNER JOIN
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INNER JOIN - produces only the set of records that match in both Table A and Table B
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FULL OUTER JOIN - produces the set of all records in tableA and tableB, with matching records from both sides where available. If there is no match, the missing side will contain null
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Device:f63d0868 d7c1 4db8 bb63 39892d705ef7124805 1l9o1i9.s5ugd86w29
LEFT OUTER JOIN - produces a complete set of records from tableA, with the matching records (where available) in tableB. If there is no match, the right side will contain null
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LEFT OUTER JOIN with WHERE - produce the set of records only available in tableA, by excluding the records we don't want from the right side using a WHERE clause
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Device:f63d0868 d7c1 4db8 bb63 39892d705ef7127908 iyfflr.pczvcp7gb9
FULL OUTER JOIN with WHERE - produce the set of records unique to tableA and tableB, by performing a full outer join and then excluding the records we don't want from both sides via a where clause
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FULL OUTER JOIN - produces the set of all records in Table A and Table B, with matching records from both sides where available. If there is no match, the missing side will contain null
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In SQL JOINS - LEFT signifies what you are selecting and RIGHT denotes what you are joining into
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FULL OUTER JOIN SELECT * FROM table1 FULL OUTER JOIN table2 ON table1.name = table2.name
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