OracleSQLの黒本ジュンク堂にないんだけど あとJavaGold高すぎもっと安くしろ

A single missing 'ON' clause just cost the company $10,000 in cloud compute fees. 😱 Most developers think a Cartesian Product is just a theoretical math concept. Until they run a 'CROSS JOIN' on two 1-million row tables and watch the production database grind to a halt. I built an Interactive Masterclass to show you exactly how to avoid this 'Oops' moment. Inside the demo:🛠️ Workbench: Break the DB (safely) with Cartesian products. 🎯 Inner Joins: The precision matches you actually want. 🛡️ Left Joins: Your safety net for missing data. 👇 Stop the crashes. Master the geometry of SQL. 🔔 Follow me @quanwen_zhao for more Oracle deep-dives! #OracleSQL #DatabaseReliability #SQLTips See my post in the comment area! 👇 Namely, like, repost are always welcome ...

➤ Important Rule - Read Inside Out! ROUND(ABS(MOD(-17, 5)), 0) Step 1: MOD(-17, 5) → -2 (innermost) Step 2: ABS(-2) → 2 (next level) Step 3: ROUND(2, 0) → 2 (outermost) Final Output: 2 • Always evaluate innermost first, work outwards! ➤ Quick Summary: • ROUND(n, d) = rounds to d decimal places, negative d rounds to tens/hundreds • TRUNC(n, d) = cuts at d decimal places, no rounding at all • MOD(n, d) = remainder after division, use to find even/odd numbers • ABS(n) = always returns positive value useful for differences • CEIL(n) = always rounds UP even 55.1 becomes 56 • FLOOR(n) = always rounds DOWN even 55.9 becomes 55 • POWER(n, e) = n raised to power e • SQRT(n) = square root • Nested functions = inner executes first, outer uses inner's result • Always read nested functions inside out • Can nest as many levels as needed 2, 3, 4 levels deep all work #OracleSQL #NumericFunctions #ROUND #TRUNC #MOD #NestedFunctions #LearnSQL #SQLBeginners #Day23 #100DaysOfCode #TechTwitter

➤ Real World RPAD Uses: -- Format report columns with consistent width: SELECT RPAD(name, 15, ' ') || RPAD(city, 15, ' ') || RPAD(dept, 10, ' ') AS report_line FROM employees; Output: Report Line Raj Delhi Finance Priya Mumbai IT Arjun Delhi Finance • Everything aligned perfectly like a report! ➤ Quick Summary: • TRIM() = removes spaces or characters from both sides • TRIM(char FROM string) = removes specific character from both sides • TRIM LEADING = left side only, TRIM TRAILING = right side only • LTRIM() = removes characters from LEFT keeps removing while left char is in list • RTRIM() = removes characters from RIGHT - same logic • Combine LTRIM RTRIM to remove from both sides for specific chars • REPLACE(string, find, replace) = replaces ALL occurrences • REPLACE with empty string = deletes the search string • LPAD(string, length, char) = pads LEFT to reach total length • RPAD(string, length, char) = pads RIGHT to reach total length • If string is longer than total_length - LPAD/RPAD truncate it #OracleSQL #TRIM #LTRIM #RTRIM #REPLACE #LPAD #RPAD #CharacterFunctions #LearnSQL #Day22 #100DaysOfCode #TechTwitter

➤ Most Important Use - Case Insensitive Search! Remember Oracle data is case sensitive. These functions solve that! -- Problem: You don't know if city is stored as -- 'Delhi', 'DELHI', 'delhi', 'dElHi' -- ❌ This might miss results: WHERE city = 'Delhi' -- ✅ This finds regardless of how it's stored: WHERE LOWER(city) = 'delhi' WHERE UPPER(city) = 'DELHI' -- Real Example — user types city in any case: SELECT name, city FROM employees WHERE UPPER(city) = UPPER('&enter_city'); -- User types 'delhi', 'DELHI', 'Delhi' → all work! ✅ ➤ Using Functions in ORDER BY: -- Sort by name case-insensitively: SELECT name FROM employees ORDER BY LOWER(name); ➤ Quick Summary: • Single Row Functions = work on one row at a time - return one result per row • Group Functions = work on multiple rows - return one result total (covered later) • Four categories - Character, Numeric, Date, Conversion • LOWER() = converts entire string to lowercase • UPPER() = converts entire string to uppercase • INITCAP() = first letter of each word uppercase, rest lowercase • Most important use = case-insensitive search using UPPER() or LOWER() in WHERE clause • Functions can also be used in ORDER BY clause #OracleSQL #SingleRowFunctions #UPPER #LOWER #INITCAP #CharacterFunctions #LearnSQL #SQLBeginners #Day20 #100DaysOfCode #TechTwitter

> Quick Summary: • &variable = placeholder that asks user for value at runtime • & asks for value every single time it appears - even same variable name • Text/Date values need quotes around & - numbers don't • & works in SELECT, FROM, WHERE, ORDER BY - anywhere in the query • &&variable = asks only ONCE and remembers for the whole session • && persists automatically - second occurrence doesn't prompt again • DEFINE variable = value = pre-sets the value - Oracle never asks • DEFINE alone = shows value of one variable • DEFINE with no args = shows ALL defined variables • UNDEFINE variable = clears it from memory - Oracle asks again next time • Professional scripts use DEFINE at top - change once, updates everywhere #OracleSQL #SubstitutionVariables #DEFINE #UNDEFINE #SQLScripting #LearnSQL #SQLBeginners #Day18 #100DaysOfCode #TechTwitter

> With Multiple Columns: SELECT name, dept, commission FROM employees ORDER BY dept ASC, commission DESC NULLS LAST; • Within each department → commission highest first → NULLs at the very end! > Quick Summary: • ORDER BY = sorts results - always the last clause in the query • Can sort by column name, alias, or position number • Column alias CAN be used in ORDER BY - it runs after SELECT • ASC = low to high (default - no need to write it) • DESC = high to low - must write explicitly • Each column in multi-sort can have its own ASC or DESC direction • Dates sorted chronologically - DESC gives most recent first • ASC → NULLs at bottom by default • DESC → NULLs at top by default • NULLS FIRST → forces NULLs to top regardless of sort direction • NULLS LAST → forces NULLs to bottom regardless of sort direction #OracleSQL #ORDERBY #ASC #DESC #NullsFirst #NullsLast #LearnSQL #SQLBeginners #Day15 #100DaysOfCode #TechTwitter

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> IS NULL Operator - Why IS NULL? We covered NULL before - let's consolidate it here completely in the context of WHERE. Use IS NULL and IS NOT NULL to check for NULL values. Never use = NULL or != NULL — they will never work! ~IS NULL: -- Employees with NO commission: SELECT name, commission FROM employees WHERE commission IS NULL; Output: Name | Commission Priya | NULL Neha | NULL Suresh | NULL Meera | NULL ~IS NOT NULL: -- Employees WHO HAVE commission: SELECT name, commission FROM employees WHERE commission IS NOT NULL; Output: Name | Commission Raj | 5000 Arjun | 3000 Kavya | 2000 Vikram | 8000 > Why = NULL Never Works: WHERE commission = NULL -- Returns 0 rows ❌ ALWAYS! WHERE commission IS NULL -- Works correctly ✅ Because NULL means unknown. Oracle can't confirm if unknown = unknown. Think of it as: "Is mystery box 1 equal to mystery box 2?" You can't know - both are unknown! > Quick Summary: • IN = match against a specific list - much cleaner than multiple OR conditions • NOT IN works - but NEVER put NULL in the list or you get zero results • Use IN for discrete specific values, BETWEEN for continuous ranges • LIKE = pattern search using wildcards • % = any number of characters (zero or more) • _ = exactly one character • Data is case sensitive - use UPPER() with LIKE for safety • ESCAPE lets you search for literal % or _ characters • IS NULL = only correct way to check for NULL • = NULL always returns zero rows - never use it #OracleSQL #INOperator #LIKEOperator #ISNull #Wildcards #LearnSQL #SQLBeginners #Day13 #100DaysOfCode #TechTwitter

> NOT BETWEEN: -- Employees whose salary is NOT between 50000 and 65000: SELECT name, salary FROM employees WHERE salary NOT BETWEEN 50000 AND 65000; Output: Name | Salary Arjun | 48000 Neha | 70000 Kavya | 45000 Vikram | 80000 > Common Mistake with BETWEEN: Always write the smaller value first! WHERE salary BETWEEN 65000 AND 50000 -Returns nothing! WHERE salary BETWEEN 50000 AND 65000 -Correct! > Quick Summary: • WHERE = filters rows - runs before SELECT in processing order • Text values need single quotes, numbers don't, dates need single quotes • Cannot use column alias in WHERE - alias doesn't exist yet • =, !=, <>, >, <, >=, <= all comparison operators • != and <> both mean not equal, <> is SQL standard • Dates compared chronologically - later date = greater value • BETWEEN is inclusive - both boundary values are included • Always write smaller value first in BETWEEN • NOT BETWEEN excludes the range #OracleSQL #WHEREClause #ComparisonOperators #BETWEEN #LearnSQL #SQLBeginners #Day12 #100DaysOfCode #TechTwitter

> DUAL Table - Quick Note DUAL is a special dummy table in Oracle with exactly one row and one column. Used when you want to run a SELECT that doesn't need a real table. SELECT 2 2 FROM dual; -- Output: 4 SELECT SYSDATE FROM dual; -- Output: today's date SELECT Q'[Raj's name]' FROM dual; -- Output: Raj's name • Think of DUAL as Oracle's calculator table - use it whenever you want to test expressions or functions without needing a real table #OracleSQL #Aliases #DISTINCT #Concatenation #QOperator #DESCRIBE #DUALTable #Comments #LearnSQL #Day11 #100DaysOfCode #TechTwitter

> NULL Summary Table: Concept Rule What is NULL Absence of any value NULL = NULL FALSE - never equal NULL = 0 FALSE - completely different Check for NULL Use IS NULL / IS NOT NULL Math with NULL Always results in NULL Replace NULL Use NVL() function NULL in ASC orderAppears LAST NULL in DESC orderAppears FIRST #OracleSQL #NullValues #NVL #IsNull #COALESCE #LearnSQL #SQLBeginners #Day9 #100DaysOfCode #TechTwitter

JavaとOracleSQL📖📚 本当にずっと親

> Quick Summary: • Data Type = tells Oracle what kind of data a column can store • VARCHAR2 → variable text (most used for text) • NUMBER → all numbers - whole and decimal (most used for numbers) • DATE → stores date time both together (most used for dates) • CHAR → fixed length text - use only when length never changes • BLOB → images, videos, files • CLOB → very large text up to 4 GB • TIMESTAMP → date time milliseconds • ROWID → Oracle's internal row address - auto created #OracleSQL #DataTypes #VARCHAR2 #NUMBER #DATE #BLOB #LearnSQL #SQLBeginners #OracleDataTypes #Day8 #100DaysOfCode #TechTwitter

> Quick Summary: • SQL = language used to communicate with a database • SQL is declarative - you say WHAT, Oracle figures out HOW • DDL → defines structure (CREATE, ALTER, DROP, TRUNCATE) • DML → works with data (INSERT, UPDATE, DELETE) • DQL → fetches data (SELECT) • DCL → controls access (GRANT, REVOKE) • TCL → manages transactions (COMMIT, ROLLBACK, SAVEPOINT) • Oracle SQL = standard SQL extra enterprise-grade power #OracleSQL #WhatIsSQL #SQLCommands #DDL #DML #DQL #DCL #TCL #LearnSQL #SQLBeginners #Day7 #100DaysOfCode #TechTwitter

7. Trigger - The Automatic Responder Code that automatically runs when something happens in the database - on INSERT, UPDATE, or DELETE. When a new order is placed (INSERT) → Trigger automatically sends a confirmation email • Think of it like a motion sensor light - you walk in, it turns on automatically • We will cover these in detail later in the course > We will learn each of these objects in deep detail as we progress. #OracleSQL #DatabaseObjects #LearnSQL #SQLBeginners #OracleForBeginners #Day6 #100DaysOfCode #TechTwitter

"It seems to me to be a reasonable approach." If you're working on some monstrosity like OracleSQL yes. It's also reasonable to expand your build system so that you can write new pieces of code in rust

🆚 CDB vs PDB - Key Differences: • CDB → one per Oracle installation, manages all PDBs, holds shared infrastructure • PDB → many per CDB, holds actual business data, fully portable • DBA connects to CDB to manage all databases at once • Developers connect to PDB to work with their specific data 🏛️ Oracle-Specific Facts: • PDB architecture was introduced in Oracle 12c (2013) • Oracle 21c made CDB/PDB the only supported architecture • In Oracle 19c and above - every database you create is technically inside a CDB • Cloud services like Oracle Autonomous Database use PDBs heavily • One CDB can have up to 4096 PDBs in Oracle 19c 🧠 Quick Summary: • CDB = Container Database - the outer shell • PDB = Pluggable Database - self-contained DB inside CDB • CDB$ROOT = master container managing everything • PDB$SEED = read-only template for creating new PDBs • PDBs share memory and processes → saves cost and resources • PDBs can be plugged/unplugged → makes migration easy • Every Oracle 21c database uses this architecture #OracleSQL #PluggableDatabase #PDB #CDB #OracleArchitecture #Oracle12c #LearnSQL #DatabaseAdmin #Day5 #100DaysOfCode #TechTwitter

🔑 How Relationships Are Built in Oracle SQL: CREATE TABLE customers ( cust_id NUMBER PRIMARY KEY, name VARCHAR2(50) ); CREATE TABLE orders ( order_id NUMBER PRIMARY KEY, cust_id NUMBER, item VARCHAR2(50), CONSTRAINT fk_cust FOREIGN KEY (cust_id) REFERENCES customers(cust_id) ); • PRIMARY KEY → marks the unique identifier • FOREIGN KEY → creates the link between tables • REFERENCES → points to which table and column the FK links to ⚠️ What Happens if You Break the Relationship? If you try to insert an order with a cust_id that doesn't exist in CUSTOMERS → Oracle throws an error. This is called a Referential Integrity Constraint - it protects your data from becoming inconsistent. 🏛️ Oracle-Specific Notes: • Oracle enforces FK constraints strictly by default • You can add ON DELETE CASCADE → if a customer is deleted, their orders delete too • You can add ON DELETE SET NULL → if a customer is deleted, cust_id in orders becomes NULL • Oracle's CONSTRAINT keyword lets you name your constraints for easier debugging 🧠 Quick Summary: • Entity = a real-world thing stored as a table • Relationship = how two tables are connected • 1:1 → one row links to one row (rare) • 1:M → one row links to many rows (most common) • M:M → needs a junction table in between • FK REFERENCES = how Oracle builds relationships • Referential Integrity = Oracle protects data consistency automatically #OracleSQL #EntityRelationship #ERDiagram #DatabaseDesign #PrimaryKey #ForeignKey #RDBMS #LearnSQL #Day4 #100DaysOfCode #TechTwitter

🛡️ RDBMS follows ACID properties - this is what makes it reliable: ↳ A - Atomicity → all operations complete, or none do ↳ C - Consistency → database always stays in a valid state ↳ I - Isolation → transactions don't interfere with each other ↳ D - Durability → once saved, data stays saved even after a crash This is why banks use Oracle - they cannot afford to lose even one transaction. 🏛️ Oracle-Specific Facts: ↳ Oracle Database is an RDBMS - it stores data in tables ↳ We use SQL (Structured Query Language) to interact with it ↳ Oracle supports all ACID properties out of the box ↳ In Oracle, every table lives inside a schema (a user) 🧠 Quick Summary: • RDBMS = database with tables connected via keys • Primary Key = uniquely identifies a row • Foreign Key = links two tables together • Relationship = how tables talk to each other • Oracle DB = enterprise-grade RDBMS • ACID = the 4 properties that keep data safe and reliable #OracleSQL #RDBMS #RelationalDatabase #SQL #PrimaryKey #ForeignKey #ACID #LearnSQL #Day3 #100DaysOfCode #TechTwitter #BuildInPublic