coalesce sql

altThe method allows us to recover from a nothing with a default maybe, but sometimes our recovery efforts might need t o Enlist some logic to recover properly. In this lesson, we'll see how we can use the method on the maybe to recover from a nothing by coalesce calling a function. When one would like option to a Maybe but would like to remain within a Maybe type, coalesce can be used.

In the ETL project often encounter such a situation:A column in the target table originates from a different source data table A,b,c. If no valid data in A is taken from B, if none in B is taken from C, then set to null if none is in C.In situations like this, sometimes the first thought is the case and then else end, which can solve the problem, but it's still more complicated and error-prone, especially when someone looks at the SQL you're writing.S

functions are general functions. Mainly: NVL,NVL2,NULLIF,COALESCE, these functions can be used on all types.The following is a brief introduction to the use of several functions.Before you introduce this, you have to understand what a null value in Oracle is NULL1.NVL functionThe format of the NVL function is as follows: NVL (EXPR1,EXPR2)The meaning is: if the first parameter of Oracle is empty then the value of the second parameter is displayed, and

example, if the double-precision real number is higher than the single one), and then converted to a higher-priority value, the type is returned. --The first parameter is number type, the second parameter is varchar type, cannot be implicitly converted, error.Sql> Select ENAME,NVL (Comm, ' no comm ') from EMP;Select ENAME,NVL (Comm, ' no comm ') from EMP*ERROR at line 1:Ora-01722:invalid number --Converts the first argument to a character type and the second parameter is the number type, then t

Coalesce is actually a useful function in SQL, and coalesce () is the primary function of returning the first non-null value;For example:COALESCE (NULL, NULL, 2, 4) ==> returns 2,COALESCE (null,null) ==> returns nullOf course, the primary function of the COALESCE () function

Ref http://blog.csdn.net/mtj66/article/details/52629876 ###################################### If usage select * FROM (select *,IF (B.name was null, True,false) as Bo from Test1 A left joins Test2 B on A.name =b.name) T3; T3.name t3.age t3._col2 t3.id t3.boLucy + NULL NULL TrueLily Lily 1 FalseJim + NULL NULL TrueHenry NULL null TrueTime taken:11.266 seconds, Fetched:4 row (s) ####################################### COALESCE UsageSELECT * FROM (SELE

PLSQL _ basic series 6_determine the operation NVL/NULLIF/COALESCE/NVL2, plsqlnvl BaoXinjian I. Summary The following functions are applicable to any data type and use null values:NVL (expr1, expr2)NVL2 (expr1, expr2, expr3)NULLIF (expr1, expr2)COALESCE (expr1, expr2,..., exprn) Ii. Case study-NVL 1. The NVL function format is as follows: NVL (expr1, expr2) 2. Meaning: if the first oracle parameter is b

ENAME,NVL2 (Comm,-1,1)FromEmp Ename NVL2 (COMM,-1,1) —————— – ————— SMITH 1allen - 1ward -11martin -1blake 1clark 1 scott 1 In the example above. The result is 1 of the original is not empty, and the result is-1 of the original value is empty.Iv. Cases-Nullif 1. The format is as follows: Nullif (EXP1,EXPR2)2. The implication is that the NULLIF function is to return null (NULL) if EXP1 and exp2 are equal, otherwise the first value is returned.3. For example:

http://blog.csdn.net/baoshan/article/details/2907602 COALESCE () Usage Select COALESCE (null,null, ' 1 ', ' 2 ') returns 1 Action returns the first NON-EMPTY expression in its argument. IsNull () Usage Select isnull (null,0) returns 0 Action encountered null return specified value Nullif () Usage nullif (' AA ', ' AA ') returns NULL, NULLIF (1,2) returns 1 Action two values equal returns null http://blo

Database Application Software often saves multiple and related entity information in the same table. For example, the purchased parts and locally produced parts are both parts, and the managers and workers are employees. Although multiple entity types are slightly different in data storage, they have many similarities, therefore, we usually use one table instead of two tables. Processing such a table usually means performing a condition test on each row to check which object type is being proc

Tips for using coalesce () in mysql Coalesce () Explanation The first non-empty expression in the return parameter (from left to right, and so on); Tips for using coalesce () in mysql Example A, B, and c variables. Select coalesce (null, 2, 3); // Return 2 Select coalesce