The MySQL database is an open source database based on structured data. The SQL statement is the core language in the MySQL database. However, in the MySQL database to execute SQL statements, you need to be careful two traps.
Trap one: null value
Difference
Link:
As a verb, it means combining the contents of two or more tables and producing a result set that merges the columns and rows of each table. Table joins generally use the data that they have in common. For example, you can
Insert operations on a table, such as two fields of datetime and int types, are allowed to be null.
With "sqlcmd." Parameters.Add ("@t12", tb12. Text) Parameter binding. DateTime type, TB12. Text is empty, insert success, do not error, view the
Apply operator
Use the APPLY operator to call a table-valued function for each row returned by an external table expression that implements a query operation. The table-valued function is the right input, and the outer table expression is
function | string
if exists (SELECT * from dbo.sysobjects WHERE id = object_id (N ' [dbo].[ P_stuff] and OBJECTPROPERTY (ID, N ' isprocedure ') = 1) drop procedure [dbo]. [P_stuff] Go
/*--ntext Field Processing
analog String Handler stuff
Variables | data | data type
What is a variable? Presumably it's a container for information (or, more specifically, a pointer to a storage location in the memory space, but it's OK to be able to understand it, so you should remember the
We discussed the database performance optimization on the other hand, that is, the use of database server built-in tools to analyze and optimize.
▲show
Perform the following command to understand the running state of the server: MySQL >show status;
First of all, the difference between null and undefined:
Performs typeof on declared but uninitialized and undeclared variables, and returns "undefined".
Null represents an empty object pointer, and the typeof operation returns "Object".
It is
Stored Procedure | function | String if exists (SELECT * from dbo.sysobjects WHERE id = object_id (N ' [dbo].[ P_stuff] and OBJECTPROPERTY (ID, N ' isprocedure ') = 1)
drop procedure [dbo]. [P_stuff]
Go
/*--ntext field Processing
Impersonation
To obtain all database information on a SQL Server using stored procedures sp_databases
Execute command in query analysis exec sp_databases
Result: Master 14464 NULL
Model 1280 NULL
msdb 14336 NULL
Northwind 4352 NULL
Pubs 2560 NULL
Store 1912
3. Data display related commands
3.1 Set separator:. Separator separator
Help ?
1
. Separator STRING Change separator used by output mode and. Import
Example Default separator is |?
1 2 3 4 5 6 7 8 9
In the initial stages of software development, developers always want to design the smallest details of the whole system and then write the code in a single thread. It takes a long time for software to develop, but developers have been doing it.
So
Variables | data | data type
Data types in Flash1, the String value example: mytext= "F4", Mytex is a string, the string is a sequence composed of characters
2, numerical example mynum=2 mynum is the numerical value, may use the mathematical
Dwwwstr= "dwww.cnwww.dwww.cn Design Home"Dwwwstr= InStrRev (dwwwstr, "")Response.Write Dwwwstr%>
InStrRevDescribeReturns the position from the end of a string that appears in another string.GrammarInStrRev (string1, string2[, start[, compare])The
recently started to study the shorthand method for JavaScript, and here's a short method for judging null Recently I found myself writing JavaScript code is bloated, so I began to study the shorthand method of JavaScript. This will make our
Transformation
1. Row and column conversion--ordinary
Suppose there is a student record (CJ) as followsName Subject ResultsJohn Language 80John Math 90John Physics 85Dick Language 85Dick Math 92Dick Physics 82Harry Math 60
Want to becomeName
The deadlock has an important effect on the performance and throughput of the system, and it is found that the deadlock of MIS is mainly because two or more threads (login) preempt the same table data resource. Causing a long time to preempt the
Lead and LAG functions in SQL Server
Lead and LAG functions
Lead
Accesses data in subsequent rows of the same result set, instead of using a self-join in SQL Server 2012. The lead provides access to the row with the given physical offset after
Statement
1. What is the maximum number of days of the month in which a date is obtained with a statement?
SELECT Day (DATEADD (DD,-day (' 2004-02-13 '), DATEADD (mm, 1, ' 2004-02-13 '))-as ' Day number '
2. Fewer records become multiple
N-r size comparison logical numbers
Nan is not a number
NE is not equal to
Create a new object
NewLine Create a new line (insert a space line)
NextFrame Run Next Frame
Nextscene Next Scene
Not a logical non-
Null null value
Number (function) numbers
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