Subquery

Subquery or Inner query or Nested query A subquery is a query within another query. The outer query is called as main query and The inner query is called as subquery. A subquery is usually added in the WHERE Clause of the sql statement.

Subquery Subquery or Inner query or Nested query Syntax The subquery (inner query) executes once before the main query (outer query) executes.

Subquery Subquery or Inner query or Nested query Two tables ‘STUDENT' and ‘MARKS' with common field ‘ID'. To write a query to identify all students who get better marks than that of the student who's ID is ‘2‘. If we know the mark of ID ’2’ then SELECT A.ID,A.NAME, B.MARK FROM STUDENT A, MARK B WHERE A.ID=B.ID AND B.MARK >70;

Subquery Subquery or Inner query or Nested query STUDENT MARK S But we do not know the marks of ‘2'. we require two queries (Nested query) One query returns the marks of '2' and Second query identifies the students who get better marks than the result of the first query SELECT A.ID,A.NAME, B.MARK FROM STUDENT A, MARKS B WHERE A.ID=B.ID AND B.MARK > (SELECT MARK FROM MARKS WHERE ID=2);

Subquery Rules for subqueries: Subqueries must be enclosed within parentheses. Inner query can have only one column in the SELECT clause, unless multiple columns are in the main query. Inner Query cannot use An ORDER BY clause, although the main query can use an ORDER BY. The BETWEEN operator cannot be used with a inner query; however, the BETWEEN operator can be used within the main query.

Subquery Subqueries can be used with SELECT INSERT UPDAT E, and DELET E statements along with the operators like =,, >=, <=, IN, BETWEEN etc.

Subquery Subquerie in select statement Tab1 Tab2 List all details from tab2, where rollno =4 in tab1 select * from tab2 where rollno in (select rollno from tab1 where rollno=4)

Subquery Subquerie in insert statement Tab1 Tab4 Consider table tab1 & tab4 with similar structure. To copy the records from tab1 to tab4 where rollno=3 insert into tab4 select * from tab1 where rollno in (select rollno from tab1 where rollno=3)

Subquery Subquerie in update statement Tab1 Tab2 Increment the marks with 10 in tab2, whose rollno = 4 in tab1 update tab2 set mark=mark+10 where rollno in (select rollno from tab1 where rollno=4)

Subquery Subquerie in delete statement Tab1 Tab2 Delete all details from tab2, where rollno is 4 in tab1 delete from tab2 where rollno in (select rollno from tab1 where rollno=4)

Subquery NULL Functions Consider the table N_TEST 3 rd and 2 nd row contains NULL values SELECT * FROM N_TEST WHERE COL1=NULL; Then result will no data found

Subquery NULL Functions Consider the table N_TEST 3 rd and 2 nd row contains NULL values Instead we must use the IS NULL or IS NOT NULL operators SELECT * FROM N_TEST WHERE COL1 IS NULL; Then SELECT * FROM N_TEST WHERE COL1 IS NOT NULL;

Subquery NULL Functions NVL The NVL function allows to replace null values with a default value. If the value in the first parameter is null, the function returns the value in the second parameter. If the first parameter is any value other than null, it is returned unchanged.

Subquery NULL Functions NVL COL1 in the N_TEST table contains null in all rows except the first & second Using the NVL function we replace the null values with ‘AJI'. SELECT COL1,NVL(COL2,'AJI') AS OUTPUT FROM N_TEST;

Subquery NULL Functions DECODE The DECODE function have 3 parameters. If second parameter occurs will replace to third parameter Other wise it return NULL SELECT COL1,DECODE(COL2,NULL,'AJI') AS OUTPUT FROM N_TEST; Here all NULL, converted to ‘AJI’ other wise it converted to NULL

Subquery NULL Functions NVL2 The NVL2 function accepts three parameters. If the first parameter value is not null it returns the value in the second parameter. If the first parameter value is null, it returns the third parameter. SELECT COL1,NVL2(COL2,'SAJI','AJI') AS OUTPUT FROM N_TEST;

Subquery Numeric Functions ABS(number) Returns the absolute positive value of an expression. SELECT ABS(COL1) AS OUTPUT FROM N_TEST;

Subquery Numeric Functions MOD(number, divisor) Returns the remainder of the division from 2 integer values. SELECT COL1,MOD(COL1,2) AS OUTPUT FROM N_TEST

Subquery Numeric Functions POWER(number, power) Returns the exponential value for the numeric expression. SELECT COL1,POWER(COL1,2) AS OUTPUT FROM N_TEST

Subquery String Functions CONCAT(string1, string2) Join 2 strings SELECT COL1,CONCAT(COL1,'AJI') AS OUTPUT FROM N_TEST

Subquery String Functions INITCAP(string) Returns a string with each word's first character in uppercase and the rest in lowercase. SELECT COL1,INITCAP(COL2) AS OUTPUT FROM N_TEST

Subquery String Functions REPLACE(string,searchString,replacement) Used to replace the string1, having string2 with string3. SELECT COL1,REPLACE(COL2,'one','TWO') AS OUTPUT FROM N_TEST

Subquery String Functions SUBSTR(string,start,length) Displays the string Starting with the character in string and select the next characters. SELECT COL1,SUBSTR(COL2,2,2) AS SUBSTR FROM N_TEST

Subquery Date Functions SYSDATE Returns the current date Tab1 SELECT SYSDATE FROM TAB1;

Subquery Date Functions DD Addition of date SELECT DOB,DOB+2,DOB+30 FROM DD;

Subquery Date Functions MONTHS_BETWEEN returns number of months between dates date1 and date2. If date1 is later than date2, then the result is positive. Syntax MONTHS_BETWEEN(date1,date2); Other wise negative SELECT DOB, MONTHS_BETWEEN('10-FEB-1998',DOB) FROM DD

Subquery Date Functions ADD_MONTHS Used to add month Syntax ADD_MONTHS(date,int); Add DOB month with 2 SELECT DOB, ADD_MONTHS(DOB,2) FROM DD;

Subquery Date Functions LAST_DAY(date1) Find the date of the last day of the month that contains date1. SELECT DOB, LAST_DAY(DOB) FROM DD;

Subquery Date Functions TRUNC(date1,’char’) Find the date of first day of the month containing in date1 when char=‘MONTH’. If char=‘YEAR’, it finds first day of year containing date1. SELECT DOB, TRUNC(DOB,'MONTH'),TRUNC(DOB,'YEAR') FROM DD;

Subquery Date Functions TO_CHAR() SELECT DOB, TO_CHAR(DOB,'DAY-DDTH- MONTH-YEAR') FROM DD;

Subquery Date Functions TO_DATE converts char of CHAR, VARCHAR2, NCHAR, or NVARCHAR2 datatype to a value of DATE datatype. SELECT DOB, TO_DATE('FEB ','MONTH DD YYYY') FROM DD;

Subquery Date Functions

Subquery Date Functions