What does mean in SQl?

In the select statement, (+) refers to an outer join, which is a method of joining a query.

Example: select t1*,t2.* from dept t1,emp t2 where

Where: means that even if there is no matching record in T2 (EMP table), it is queried in T1 (DIPT table).

The SQL language is a structured query language.

Structured Query Language).

The SQL language is a database query and programming language.

for accessing data and querying, updating, and managing relational database systems; It is also an extension for database script files.

SQL statements used in Oracle databases can be added after the table (+) to implement an outer join.

The query result is based on the primary table, that is, if there are no records in the slave table that meet the conditions of the primary table, blank rows are displayed to join the primary table.

For example, Tablea A, Tableb B

The result of the query is:

First, the reference is different.

1. *: Query all data columns in the Qinxin database.

2. %: indicates the number of rows that match the conditions specified in the database.

Second, the usage is different.

1. *: used in the query column name part of the SQL statement, which is located at the front end of the SQL statement.

2. %: used after the conditional statement of the SQL statement and after the where statement.

Third, the rules are different.

1. *: You can use the * sign instead of the column, or you can use the column name instead, not Chashou Mountain must use *.

2. %: The wildcard must be used with the like operator.

It is not equal to 0, <> is a kind of relational copy operator, and the commonly used relational operators are: , less than, e.g. a<=, less than, e.g. a<=b>, greater than, e.g. a>b, greater than or equal to, e.g. a>=b, equal, e.g. a==b, not equal, e.g. a!=b

That is, it is not equal to 0, <> is a kind of relational operator, and the commonly used relational operators are (greater than),

Less than), = (greater than or equal to),

(less than or equal to).

equal), this different database is also written differently, some are ==, some are =!= (not equal).

(not equal), not equal to two ways to write.

It's just a condition, such as select * from the table name where field <>0

This usually means binding a variable. Here's how:

1. First of all, the most basic assignment is the default value of the mu, as shown in the figure below.

2. Then you can also query the data table to assign a value to the variable excavation and elimination, as shown in the data table shown in the figure below.

3. You can directly assign values to variables through the select statement, as shown in the following figure.

4. After the assignment, you can output it to see the result, as shown in the figure below.

5. Then you can see that the result is the same as the value of the data table, as shown in the figure below, and the bridge macro is completed.

SQL statementA symbol means a connection, which is equivalent to a connector in a string.

Common symbols in SQL:

1, = means equal.

2. <> means that it is not equal to.

3. > means greater than.

4. < means less than.

5. >= indicates greater than or equal to.

6. <= means less than or equal to.

7. between means within a certain range.