1. Multi-table queries Subqueries
DBMAN 4
Multi-table queries
Subqueries
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2. SELECT Displayed order of suffixes
INTO
FROM
WHERE
GROUP BY
HAVING
UNION/MINUS
INTERSECT
ORDER BY
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3. Multi-table queries Subqueries Examples
DBMAN 4
Multi-table queries
Subqueries
Examples
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4. SET OPERATORS [query] [set operator] [query]
Important: two queries with same number of columns!
Operators: UNION / UNION ALL, INTERSECT, MINUS / EXCEPT
E.g.: SELECT min(sal) as ThreeValues FROM emp UNION SELECT avg(sal) FROM emp UNION SELECT max(sal) FROM emp;
MySQL: MINUS = LEFT JOIN + WHERE … IS NULL
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5. Table structure diagram
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6. Querying multiple tables
SELECT * FROM emp, dept;
SELECT * FROM emp, emp;
SELECT * FROM emp a, emp b;
SELECT a.empno, a.ename, b.empno, b.ename FROM emp a, emp b;
SELECT a.ename, a.deptno, b.deptno, b.dname FROM emp a, dept b;
„Cross Join”  Cartesian product EMP  DEPT
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7. "MANUAL JOIN"
SELECT a.ename, a.deptno, b.deptno, b.dname FROM emp a, dept b WHERE a.deptno=b.deptno;
Works – every record has a pair!
SELECT a.empno, a.ename, a.mgr, b.empno, b.ename, b.mgr FROM emp a, emp b WHERE a.mgr=b.empno;
Where is King???  no pair found!
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8. JOIN Goes into the FROM part of a query
SELECT * FROM table1, table2  SELECT * FROM table1 JOIN_EXPRESSION table2 JOIN_CONDITION
Joining tables = connecting the foreign keys to the primary keys
Joining indexed fields is fast, joining non-indexed (non-key) fields is very slow  good-to-avoid, cannot-always-avoid
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9. NATURAL / INNER JOIN
Same result as with "MANUAL JOIN"  Only shows the records with valid pairs
SELECT * FROM emp NATURAL INNER JOIN dept; -- Not in MySQL/TSQL!
SELECT * FROM emp NATURAL JOIN dept; -- Not in TSQL
SELECT * FROM emp INNER JOIN dept using (deptno); -- Not in TSQL
SELECT * FROM emp INNER JOIN dept on (emp.deptno=dept.deptno); -- Works everywhere!
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10. LEFT JOIN … ON …
It must be used if we want to display the records WHERE there is no matching primary key record
The nonexistent records' fields will be filled with NULL values  use IFNULL/ISNULL/NVL if needed
SELECT a.ename, a.deptno, b.deptno, b.dname FROM emp a LEFT JOIN dept b ON a.deptno=b.deptno; -- No difference here, as every worker has a department
SELECT a.empno, a.ename, a.mgr, b.empno, b.ename, b.mgr FROM emp a LEFT JOIN emp b ON a.mgr=b.empno; -- KING is shown!
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11. LEFT/RIGHT JOIN … ON …
SELECT a.ename, a.deptno, b.deptno, b.dname FROM emp a RIGHT JOIN dept b ON a.deptno=b.deptno;  OPERATIONS (40) is shown as well!
Show everyone as a boss, even those who are no bosses  list unpaired records from the other side!
SELECT a.empno, a.ename, a.mgr, b.empno, b.ename, b.mgr FROM emp a RIGHT JOIN emp b ON a.mgr=b.empno;
SELECT a.empno, a.ename, a.mgr, b.empno, b.ename, b.mgr FROM emp b LEFT JOIN emp a ON a.mgr=b.empno;
 The LEFT JOIN is more used
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12. JOIN / ORACLE ONLY SELECT * FROM emp d, emp f WHERE d.mgr=f.empno (+);
SELECT * FROM emp d LEFT JOIN emp f ON (d.mgr=f.empno)
SELECT * FROM emp d, emp f WHERE d.mgr(+)=f.empno;
SELECT * FROM emp d RIGHT JOIN emp f ON (d.mgr=f.empno)
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13. FULL JOIN LEFT JOIN + RIGHT JOIN
SELECT a.empno, a.ename, a.mgr, b.empno, b.ename, b.mgr FROM emp a FULL JOIN emp b ON a.mgr=b.empno;
[The standard names this type as UNION JOIN; unavailable in MySQL…]
 Use the "MANUAL" JOIN and the LEFT JOIN
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14. BAD JOIN???
SELECT ename, sal, dname FROM emp, dept WHERE emp.deptno=dept.deptno;
SELECT ename, sal, dname FROM emp, dept WHERE emp.deptno=dept.deptno AND ename like '%E%';
SELECT ename, sal, dname FROM emp, dept WHERE emp.deptno=dept.deptno AND ename like '%E%' OR sal<3000;
SELECT ename, sal, dname FROM emp, dept WHERE emp.deptno=dept.deptno AND (ename like '%E%' OR sal<3000);
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15. JOINING TABLES
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16. JOINING TABLES
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17. JOINING TABLES
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18. JOINING TABLES
SELECT * FROM emp worker, emp boss, dept work_dept, dept boss_dept WHERE worker.mgr=boss.empno AND worker.deptno=work_dept.deptno AND boss.deptno=boss_dept.deptno;
Good, if we don't want to see the unpaired records
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19. JOINING TABLES
SELECT * FROM emp worker LEFT JOIN emp boss ON (worker.mgr=boss.empno), dept work_dept, dept boss_dept WHERE worker.deptno=work_dept.deptno AND boss.deptno=boss_dept.deptno;
NOT CHANGES ANYTHING
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20. JOINING TABLES
SELECT * FROM emp worker LEFT JOIN emp boss ON (worker.mgr=boss.empno) LEFT JOIN dept boss_dept ON (boss.deptno=boss_dept.deptno) LEFT JOIN dept work_dept ON (worker.deptno=work_dept.deptno);
PERFECT
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21. JOINING TABLES
After the join is made in the FROM/WHERE, the query can be built up just like as if it was a single-table query. Every suffix can be used, the result-table can be used the same way as with a single-table query
PRACTICE and STRUCTURED QUERIES are required!!!
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22. EXAMPLE
Display every boss' name and the average salary of those sub-workers whose salary is bigger than 1000 USD.
Only display those records where this average is smaller than 5000 USD.
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23. EXAMPLE SELECT avg(a.sal) as AVERAGE, b.ename as BOSS
FROM emp a, emp b
WHERE (a.mgr=b.empno) AND (a.sal>1000)
GROUP BY b.ename
HAVING avg(a.sal)<5000
ORDER BY AVERAGE desc;
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24. EXAMPLE SELECT avg(a.sal) as AVERAGE, b.ename as BOSS FROM emp a
INNER JOIN emp b ON a.mgr=b.empno
WHERE a.sal>1000
GROUP BY b.ename
HAVING avg(a.sal)<5000
ORDER BY AVERAGE desc;
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25. Multi-table queries Subqueries
DBMAN 4
Multi-table queries
Subqueries
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26. SUB-QUERIES
Basic principle: in some parts of the main query (field list, WHERE suffix, FROM suffix) there is another query instead of a simple field name/expression/constant/ table
 sub-query
MySQL only supports it with the newer (>4.1) versions (with some angry limitations…)
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27. Sub-queries in the field list
You can put a constant in the field list  you can put any sub-query there that returns with exactly one value
SELECT sal, (SELECT max(sal) FROM emp) as maxSal, (SELECT max(sal) FROM emp)-sal as deltaSal FROM emp ORDER BY deltaSal desc;
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28. Sub-queries in the FROM (inline view)
You can put any table in the FROM  every query returns a table  almost any sub-query can be used in the FROM, that we can use as a table in that query
The sub-query must have an alias and must be written between parentheses
SELECT * FROM (SELECT a.empno, a.ename, a.mgr, b.empno, b.ename FROM emp a, emp b WHERE a.mgr=b.empno) sub ORDER BY ename asc; Field names must be unique – use an alias!!!
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29. Sub-queries in the FROM
SELECT bosses.work_name as Worker, bosses.boss_name as Boss
FROM (
SELECT a.empno as work_id, a.ename as work_name, a.mgr as work_bossid, b.empno as boss_id, b.ename as boss_name
FROM emp a, emp b
WHERE a.mgr=b.empno) bosses
ORDER BY Worker asc;
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30. Sub-queries in the FROM
SELECT emp.deptno, min, ename
FROM (
Select deptno, min(sal) as min
From emp Group by deptno
) minimums, emp WHERE emp.sal=minimums.min and
emp.deptno=minimums.deptno;
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31. Sub-query in the WHERE
Use constant values in the WHERE  use any sub-query that returns with exactly one value
SELECT ename, sal FROM emp WHERE sal>(SELECT avg(sal) FROM emp);
SELECT ename, sal FROM emp WHERE sal=(SELECT min(sal) FROM emp);
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32. Sub-query in the WHERE
ALWAYS VERY SLOW  SUBQUERY IN THE FROM IS BETTER
SELECT ename, sal FROM emp WHERE sal>(SELECT avg(sal) FROM emp);
SELECT ename, sal FROM emp, (SELECT avg(sal) as avgSal FROM emp) sub WHERE sal>sub.avgsal; (No JOIN  caresian product, but the subquery only has one row  it simply adds the one avgSal field to emp)
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33. Sub-query in the WHERE
Use lists in the WHERE  use any sub-query that returns with exactly one column
Operators: [NOT] IN / EXISTS, ANY, ALL
Usually used with a subset:
SELECT sal FROM emp WHERE upper(job)='SALESMAN';
SELECT sal FROM emp WHERE deptno = 10;
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34. Sub-query in the WHERE
SELECT ename, sal FROM emp WHERE sal IN (SELECT sal FROM emp WHERE deptno = 10);
SELECT ename, sal FROM emp WHERE sal NOT IN (SELECT sal FROM emp WHERE deptno = 10);
Correlated sub-query
STILL SLOW!  Subquery into the FROM!
IN  INNER JOIN NOT IN  LEFT JOIN + WHERE xxx IS NULL
Optimization is not important this semester
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35. Sub-query in the WHERE
SELECT ename, sal FROM emp WHERE sal> ANY (SELECT sal FROM emp WHERE deptno = 10);
SELECT ename, sal FROM emp WHERE sal>(SELECT min(sal) FROM emp WHERE deptno = 10);
 Same results
 Oracle does an internal sort with the ANY!!!
 STILL SLOW!  Subquery into the FROM!
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36. Sub-query in the WHERE
SELECT ename, sal FROM emp WHERE sal> ALL (SELECT sal FROM emp WHERE deptno = 30);
SELECT ename, sal FROM emp WHERE sal>(SELECT max(sal) FROM emp WHERE deptno = 30);
 Same results, No automatic sort
Always use the ORDER BY, if required
STILL SLOW!  Subquery into the FROM!
Subquery should always go to the FROM … But how to avoid loooooooooooooooong queries?
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37. Fetch hierarchy in one query
Oracle> SELECT level, empno, ename, mgr FROM emp START WITH empno=7839 CONNECT BY mgr=prior empno ORDER BY level desc;
Standard, TSQL> no such way… TODO: recursive CTE …
CTE = SUBSTITUTE a subquery with a name, usable only in the main query where the CTE is defined
Similar object: views = STORE a subquery with a name, which is later usable in any main/subquery
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38. CTE/VIEW
CTE = Common Table Expression WITH xxx (columns) AS (SELECT …) SELECT * FROM xxx INNER JOIN otherTable …
VIEW: GO CREATE VIEW xxx (columns) AS (SELECT …); GO SELECT * FROM xxx INNER JOIN otherTable …
CREATE VIEW must be the only statement in the batch
The column list is optional in both cases
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39. CTE Example (AdventureWorks)
Task: which are the orders with the biggest ratio compared to the monthlyTotal?
Pre-solution: monthly totals
SELECT SUM(TotalDue) as monthlyTotal, datepart(month, OrderDate) as month
FROM Sales.SalesOrderHeader
GROUP BY datepart(month, OrderDate)
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40. Ratios with CTE
WITH statsMonthly AS (SELECT SUM(TotalDue) as monthlyTotal, datepart(month, OrderDate) as month FROM Sales.SalesOrderHeader GROUP BY datepart(month, OrderDate)) SELECT sh.SalesOrderID, sh.OrderDate, sh.TotalDue, sm.monthlyTotal, sh.TotalDue/sm.monthlyTotal as ratio FROM Sales.SalesOrderHeader sh INNER JOIN statsMonthly sm ON (datepart(month,sh.OrderDate)=sm.month) ORDER BY ratio desc;
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41. Ratios with VIEW
IF object_id('statsMonthly', 'V') is not null DROP VIEW statsMonthly; GO CREATE VIEW statsMonthly AS (SELECT SUM(TotalDue) as monthlyTotal, datepart(month, OrderDate) as month FROM Sales.SalesOrderHeader GROUP BY datepart(month, OrderDate));
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42. Ratios with VIEW
SELECT sh.SalesOrderID, sh.OrderDate, sh.TotalDue, sm.monthlyTotal, sh.TotalDue/sm.monthlyTotal as ratio
FROM Sales.SalesOrderHeader sh INNER JOIN statsMonthly sm ON (datepart(month, sh.OrderDate)=sm.month)
ORDER BY ratio desc;
CTE or View?
Used once vs used many times
Recursive CTEs are allowed – Recursive views are not always allowed (Postgresql, Teradata)
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43. Recursive CTE – Fetch whole hierarchy
WITH recQ AS (SELECT empno, ename, mgr, 0 as wl FROM emp WHERE ename = 'KING' UNION ALL SELECT nplus1.empno, nplus1.ename, nplus1.mgr, recQ.wl+1 as wl FROM emp as nplus1, recQ WHERE recQ.empno = nplus1.mgr) SELECT ename, empno, mgr, wl FROM recQ
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